I'm an old man who's new to two things; starting plants from seed and this forum :-) I came here hoping someone will help me understand something. I've read that I should cover the seed tray with the plastic dome. However, in my mind, covering the tray could be a mistake. I would think the cover might tend to maintain too much humidity as well as being detrimental to good air circulation. I can understand why I would want a certain amount of humidity after the seeds germinate and develop leaves. However, I can't understand the value of using the cover before that time.
So, here's my question... If I make sure the mixture does not dry out, wouldn't it actually be better to not use the plastic cover?
I have been starting seeds in my basement under lights for over 10 years. I use the domes untill I have good germination and then vent the dome by raising it with clothes pins like blomma does, for about 2 or 3 days then remove it all together. If the seedlings touch the dome before that time I remove it.
I also found that using a good reliable seed starting mix eliminated the damping off problem that I faced when I first started growing from seed.
Sow_sow, have fun, it is a wonderful past time and very rewarding.
Hi blomma and ladygardener1 and thanks for the quick replies. It's nice to know that folks have had good success both with and without using the dome covers. I'm sure it depends a lot on how careful we are about not over-watering. I think, to get started, I'm going to try without the use of the cover. If keeping the soil mix at the proper moisture level becomes a problem, I'll add the cover and try the clothes pins trick. Thanks again for your helpful answers, I really appreciate you taking the time to answer my post.
I do the exactly the same as ladygardener1. Since most seed starting mixes have a lot of peat in it, the soil surface can go from damp to crusty parched within no time, especially under the heat of the lights. Once germination has occurred and seedlings have formed the 1st set of leaves, the clothes pin venting works great and then remove all together in a few days. Since I out most of the day at work, it is not a worry that the surface will dry out at the critical period of germination. If you spray/mist the seedlings with chamomile tea, it will prevent damping off. Some seedlings like petunias or aster are more susceptible to damping off. Cool wet conditions are contributors to damping off. I have not had fatalities from damping off, but I have always used a preventative spray.
Joanna, do you use tea bags of Chamomile? I found Bigelow herb teas bags Cozy Chamomile. How many tea bags to a qt. of water do you use?
I don't have problems with damping off either, but was plagued with fungus gnats last year, I bought some mosquito dunks to try this year incase the gnats appear this season too.
Sow_sow, no problem we love Baby seed starters. You have to start somewhere.
Hi joannabanana and thanks for your much appreciated input! I've also printed out the information regarding seed starting that you posted below. My only experience with starting from seed was last year. I started about a dozen Hardy Hibiscus in those little peat pots. Only one survived :-( However, it was a fun experience and enough to make me want to try again this year with some annuals. I bought some seed trays (lights and heat mats too) but since my main problem last year was mildew/mold, I guess I'm a little apprehensive about keeping the covers on them. Please tell me, if I do use the covers, should I see signs of moister (on the underside of the covers) or is that an indication that the mixture is too wet?
One of the things that may have contributed to your mold problems was the peat pots themselves. Many have posted to this forum their woes using peat pots and trying to keep enough moisture in them without having severe mold problems. I would never use peat pots or those expanding disks, the problems that can arise from their use far outweighs any benefits. Try to find some good fine seed starting soilless media and you will be much happier with your results. I won't get into favorite brands of soiless media as that is another discussion that occurs here periodically. Search around and you can find lots of discussion on everyone's favorite and the advantages/disadvantages of each.
My perspective on covering/not is I always use plastic domes for starting seeds and I grow in the neighborhood of 1500 plants annually from seed. I moisten all my media, fill trays, seed and then cover my trays. Using covers helps to maintain proper moisture without having to water/mist before germination/emergence occurs. I almost never have to add moisture before emergence, unless I am germinating something that takes months to germinate at higher temperatures. (another note, I don't mist any of my seed starting media, I water everything from below) Providing alot of moisture through misting while germinating seeds can create a wet and dry stratification of the planting media which can cause problems later in plant growth. Misting/top watering can also promote development of mold/damping off (as does overwatering in general). I think it is much easier to get the moisture correct while mixing planting media and then leave it alone while seeds germinate.
If you have the correct moisture, you should just see a small amount of condensation on the plastic domes. If you see drops forming or water running down the sides of the dome, you have too much moisture and should provide some venting as mentioned above. Like others have mentioned, I will leave the top on (allowing room for air flow) until the seedlings start touching the dome.
Finally, you will get as many opinions on how to do something as there are people posting, and in general, no one is wrong. There are many different ways to grow plants from seeds and it comes down to what works for you, what supplies you have available locally (or are willing to pay to have shipped) and what kind of space/environment you have to grow plants in.
I hope some of this information helps, and if not, you know where to ask more questions ; )
Thank you so much; your post is extremely helpful to me! I'm so glad you told me about the problem with using peat pots!!! I just bought some a few days ago, so back they go! I was going to use them to transfer new seedling into (when the seedlings outgrew the seed trays).
Regarding the moisture question, I knew I was supposed to keep the soil mixture 'moist' but I wasn't exactly sure what constituted "moist" :-) Your answer was perfect! I now understand and will use the tray covers with confidence. Thanks again trc65, you have been sow, sow, helpful!
sow_sow If you want to do it cheaper, forget the trays and stock up on regular kitchen paper towels.
Last winter I decided to test seeds I have had since 2002, 2004, and 2006. Took my son's advise and used a modified Deno method. This method gave me a much higher germination count than the oldfashion way of sowing in soil, even though the seeds were old. Hardy Hibiscus germinated in 2 days after a nicking and soaking first. You can't beat that.
Here is how:
Modified Deno Method
Cut a kichen paper towel in half, then wet it. Squeeze out the excess water. Fold it in half. Place the seeds in a corner and fold one end over the seeds. Place this package into a ziplock bag and zip it, leaving a small opening to blow air into the bag to fill like a balloon. Once filled, zip it closed.
If the variety of seed you sowed require stratification, place in fridge for 2 to 3 weeks. If not skip the fridge and place in room temperature to germinate.
Sprouting time depends on variety. Check the seeds daily, starting after the 3rd day. Use a tweezer (grasp the seed casing) of those that have a radical (tiny root forming). Transfer to seed flat or pot. Make a hole with a pencil and guide the root into the hole. Plant at recommended depth, spacing 1" or more, apart. If the roots have grown into the paper towel, just tear the towel around the roots and plant it. Do not try to remove the roots from the towel.
Until the sprouted seeds have broken through the soil, they do not need light. However, once they do, grow them in a sunny window, under light, or place the flat outside in a protected area if weather is warm.
Small or tiny seeds
This method won't work for very tiny seeds, which are difficult to handle individually. You can try stratifying these seeds by mixing them in a container with moist peat moss. After recommended stratification time, scattered the peat/seed mix over the surface of seeding mix or potting soil and gently pat it down to contact the soil. Keep in room temperature for germination to begin. The only drawback to this method is that you can't tell how thickly you're sowing the seeds.
This is a no fuss way, and you are in control on how many plants you want without wasting seeds.
Below is my hardy HIbiscus seed from 2004 (still in the kitchen towel) that sprouted in 2 days after I nicked, and soaked them overnight in hand hot water. These are commercial seeds. When I sowed them in soil in 2004, only one sprouted.
Wow, thanks for that blomma! I will definitely give that method a try. Since I would only need a few of each kind of plant I decide to grow, that method sounds like a real winner to me. I'm wondering if that method would work on seeds that take a long time to germinate (such as Lantana)?
Blomma - Thank YOU! Also, thank goodness I found this thread as I was wondering on the easiest, most dummy-proof way of starting seed. This will be my first year of starting seed, so I'm completely clueless! I am so antzy about getting started, but I know that we have a few more months of cold before anything can go outside. Looking out the window is SOOOOOO deceiving here! Looks bright & sunny & warm, until you step outside. BRR!! Plus my ground's still frozen solid. :(
sow_sow, and skiekitty You are both welcome. I keep posting the Deno method in every appropriate forum since I think it is the most perfect way to sow seeds.
As far as Lantana seeds, I'm not familiar with starting it from seeds. However, the Deno method should work for all seeds, including vegetables. I started Clematis 'Radar Love' and that one took 2 months to germinate with stratification.
Will now post links to seed sowing data base I have found on the Internet that I use.
Here is the first one:
You are so right about there being as many opinions as there are people on seed sowing.
I have no idea how many plants I have started from seed over my life time...and I still enjoy playing with the whole thing. Right now I have about 300 seedlings in the basement and jugs outside that are winter sown.
Does anyone have a particular dome or hot house they like? I'm trying to find an inexpensive one that is reusable year after year.
I've used the APS starterkit from http://www.gardeners.com and they were about $50+shipping for a kit that included 2 sprouting trays with resevior and a plastic lid over it as well as a bag of dirt and seed fertilizer. It was a good deal at the time since it was my first year mass producing (200+) seedlings. But one container did not survive at the end of the year and the other one's lid is cracked because its thin plastic.
Since I'm a college student, I would really like to make a good investment into a seed starter system that I can use over and over, a bonus feature would be if the seed slots were deeper than two inches. I know there is the deep-root seedstarting system by http://www.gardeners.com for $29.95 but I just wanted to know is there a better deal out there that google hasn't showen me yet.
One thing to keep in mind is that some seeds do require light to start. I have found that I get better results with seeds needing light by using a dome over planting media with seeds pressed lightly onto the surface. The dome helps to maintain moisture at the surface of the media. This is helpful with ground cherries (Physalis species), lingonberry, blueberry seeds, etc. Putting these in baggies in the fridge has not worked well for me. Seedlings started under a humidity dome may need careful hardening off to drier air conditions and the longer they are kept under the dome, the more careful you need to be with the hardening off. The paper towel/baggie method was the only way I found to successfully germinate skirrett. I generally use a good seed start mixture under a dome and move to the paper towel/baggie if the seeds do not respond well to the first method.
ferrona... I had invested in some of those "fancy" kits. I went back to the old try and true, cell packs, trays and domes. I order new cell packs each year if I am running out, they are not that expensive. I order the seed starting mix from Gardeners Supply. This work well for me and I get very good results.
You really don't need fancy or expensive items for seedsowing. I have used about every container I can find, including the clear plastic that ground meat comes in. Just added drainage holes.
I have also used clear plastic shoeboxes purchased cheap at Walmart. Seeds don't care where they are sown as long as the container has drainage holes. Most important is the seed sowing mix.
I no longer sow seeds in a mix. I use the towel/baggies method, except for tiny seeds.
Below is a great item I purchased from Park Seed Co. a few years ago. It is 3 ft x 3ft and holds many potted seedlings once they outgrow my light fixtures. Comes with a shade cloth and zippered opening on both sides. Even with our strong winds, the 12" stakes kept it from blowing away. It folds up nicely to be stored in its own bag. I may put a heating cable in there this year so I can put the plants out earlier.
More great ideas from Wyoming. Thank you very much. We have a new very large hydroponic store in town. About 15 miles from me. It is amazing how many Hispanics are growing vegetables. I cannot believe the DEA is not parked out front. But they have great things and their Super Thrive was cheaper than any place in town.
I have used a plastic shoe box to sprout my basil last year. I guess I was looking for something that kept the plants seperate that I could use year after year. I'm trying to plan better than this past year where i sprouted all my seed packets (500 seeds). Live and learn right? :P
Right now I am using recycled blueberry containers with plastic bags saved from purchased produce over the last few months. I sterilized the containers in water with bleach added. Then I added jiffy mix with a bit more perlite and vermiculite. For added warmth I use an old heating pad wrapped in two layers of towels. So far, I have just started with this experiment. I have successfully germinated alyssum seeds from 1997, well that is what the package said. This was just after two weeks. I also used chamomile tea for watering as I heard that it prevented damp off fungus.
Any thoughts? I will be trying tomato seeds tomorrow, and I do have a lot of old ones. Anyone else have old seeds that they could not bear to throw away? If not, I might have some to spare...
I just started my seedlings under grow lights 5 days ago, I already have about a 90% germination rate (in domes) and many of the plants have already reached 1-2 inches. Should I immediately remove the domes? I noticed that a few of the plants have water on their leaves and a couple have fallen over, I want to be sure to avoid the leggy and fungus things everyone has been talking about, thanks.
Another good reason for using the domes is to keep the temperature of the soil consistent and warmer. If the soil is exposed to the air the soil mixture will cool off and a lot of seeds will not germinate.
Salad bar containers work great for seeds too. Soil in the bottom (w/holes for drainage)--holes in lid for ventilation---comes in all sizes--can be kept separate from each other--and you can see through it. .
For larger domes--you can use the plastic lids that come on large party platters, or cakes. These can also be purchased at a party supply store.like "Cake Cottage".
I will be starting seeds in about a week. No domes so far--I have just used cell packs. Sometimes I put sticks in the corners and cover the seed trays with plastic wrap until germination.
I try to get by el-cheapo! I do not grow too many seeds--just enough for my small garden.
I am with you, Gita. So far I have not purchased anything but peat, perlite and vermiculite, though I had some leftover Jiffy Mix from other years, and maybe other people??I do not recall buying any...but my memory ain't so good at 68. I have used the home-saved containers as well as a couple of windowsill Jiffy starter kits...well, OK, I did buy these, they were under $3.00 a piece. I really did not want them, but I needed the trays. They (HD) didn't just have trays. I will have to check in at my local nursery. I notice that the jiffy mix is ver hard to wet, it might as well be straight peat. I add perlite and vermiculite to it.
I bought 2 bags of MG Seed Starter Mix ans opened the bag--poured 2 quarts of warm water in it, sealed it back up, mushed it a bit and it will all be nicely moist when i start.
Trays can be anything...especially recyclable are meat trays. The deeper ones, like ground meat comes in, are especially good. The "family sized, packaged meats come on much larger trays. Also good.
Foil cookie sheets...roasting pans, etc are cheap and good.
Above, I mentioned the high, clear covers deli trays come with. I should have added that if you can get the tray AND the cover--you gave it made!
Next time you are at a catered office party--rescue all these from trash!...:o)
I used (just this morning) recycled to-go containers I've been saving up over the winter to start seeds in. They have clear lids and black bottoms (a couple are actually styrofoam bottoms) so they act as a greenhouse.
Kudos to you, jlj!
WHY spend money when you can recycle everything you need for close to FREE?
I drive people crazy with my beliefs that everything HAS a second life--somehow.
Today, I bought a 2lb container of red grapes at Safeway because it comes in this high, top/bottom perforated container...All kinds of ventilation.
I could already see it all--soil and seeds in the bottom---snap it shut--VOILA! Mini Greenhouse!
Grapes will get eaten--NO problem! Good price too! $3.99 for 2lbs!
That's what I am using right now...I also use the thin plastic bags that you put produce in to cover them intitially. After they germinate, I will take them off completely. I take them off for part of the day, so they get some air circulation, then put them back on, to keep the moisture in...oh, so easy! I did, however, have to line the bottoms, since they have holes on the sides near the bottom...I used a bit of newspaper...so far - so good...not to much of the starter mix comes out. I am also learning about seed-starting mix, as I had a couple of bags of Jiffy Mix sitting around as well as some peat pots - those are just for special cases, as most will transplant if they are small enough. I already have peat, perlite and vermiculite, so I add to it, since I do not like the way that mix does not like to absorb water. I think when I use up what I have, I will just use MG...I could not find any that did not already have fertilizer in there, as Carolyn says it is better not to have fertilzer until they have at least their true leaves on. I suppose I could put the seed-starting mix on top, and the MG on the bottom layer...timesaver tip! Oh, yeah! Still, some may leach out...not sure. I just might put the peat in the garden beds, and use MG for starting, once I get my "feet wet"! We'll see. OK, folks...bring on those money and time-saving ideas...we all need them.
I bought MG Seed Starting mix this Spring. I generally like their products.
One would think that in their Seed Starter mix there would not be much fertilizer at all! I will have to read the ingredients on the bag.
Good idea about the newspaper, evelyn--that should work fine. What if one pressed the soil down hard--at just the sides--so it would not leak out?
For bottoms of pots--I use coffee filters.
Gita, great idea...I like the idea of coffee filters. I was thinking of using used (unscented) dryer sheets, after soaking them in bleach/water, making sure any chemicals in them would come out...I have saved a few, but had to go to town today, and we did not get home until 5:40PM! Whew...I'm worn out. Tomorrow it is supposed to snow again, so it will be another good seed-starting day. I can't always sow something every single day. I did however, procure 2 Jiffy "mini-greehouses" at a discount so I only paid a couple of bucks for the larger ones...it's hard to find them without soil or peat pellets in them, and there were only 2. I wasn't even going to buy them, but I have so many seeds that need starting, so I can use them. I also picked up a huge perlite at HD, as they were out of vermiculite. I have used up all of the Jiffy-Mix , and the MG potting soil, as well as the Supersoil potting soil. So I will mix what peat I have with perlite, vermiculite and sand...different proportions for different needs.
I still have a laundry basket full of the small fruit containers, like the one you got that the grapes came in. They are various sizes, as some had blueberries in them, and they were much smaller. Maybe I can use some of them for WS'ing outside. The smaller ones might blow around if we get any high winds...then those other types of bottles make sense, as the weight of them keep them more stable. Any of the seeds that need stratifying would be ideal to WS. Also, I have a very small frig outside, on the back porch, where I have my seed-starting table (an old table that is currently not being used for anything else..)...not that it isn't cold enough...but for those who like darkness, I can do Deno in there...(seeds in paper towels or vermiculite in plastic bags...) I have a lot of work ahead of me. (That huge perlite should last me all growing season...I could even put some in my hard, clay soil, though the main purpose is for seed-starting mix.) HD said they were going to get more vermiculite.
One advantage of the smaller containers, especially for me in the beginning was for me to make mistakes. I messed up just one on the germination starting, as I took off the plastic bag too soon and took it off the heat as well, so some that were just beginning to come up croaked. It was too cold and probably dried out too much as well. That was on my first experiment on germinating old seeds...some purple alyssum that I had several packages of, so just a small loss.
Now the tomatoes are another story...the ones that looked good, I did not even wait to get them in there true leaves, and they are just fine, after being tranplanted to their cell-packs. Now I need to start my peppers, as I am behind in those. Fortunately I have only a few varieties to do.
Gita..are you using store-bought plastic wrap, or the free produce plastic bags? I don't have enough plastic wrap, I think we are out of it, so I have used the free produce bags.
I am surprised that you can still buy Vermiculite!!!! It has been off the market here for years, when it was deemed to contain asbestos. I work at a HD--and we have not sold it for 5 years or more...
I do not do so many seeds that I need to worry about a roll of Seran Wrap. Usually, I don't cover my seedlings at all.
I also do not have any heat mats---but I do put my seeded cell packs on the very lowest shelf on my light rack, which sits right above a floor heat vent. So--I hope some of the heat gets near the seed trays.
I have a very small seed rack set-up and only use 3' lights on 2 shelves. It is by a S. facing window in my bedroom.
That is all I have--and, need.
We live about 35 miles from Sacramento, and I think they might have an IKEA there...and we are 20 mountain miles to the nearest town...Placerville, but they have a HD. There are loads of warnings on the bag about using a mask when using it. (the perlite), as that one is lighter than vermiculite and will spew out dust when you handle it. The vermiculite does not do that. The HD is out of trays, but I don't know if they will get more. I have not yet went to my local nursery. Several nurseries went out of business, but there is one that is still there. They are in a neighboring town, but still at least 25 miles away.
Hah! You have your seedlings in your bedroom, too! I thought that I would be able to keep a better eye on them, as well there is no other place that I can put them inside the house, but I have a rack with shelves, outside on the back porch for putting cuttings and seedlings that I have WS'd. I am also putting some in the downstairs (cold) bathroom window, for those who like cold germination...the heat is upstairs where I can watch them...they grow so fast. I take them off the heat, once they are up well, either with one set of true leaves or in the case of tomatoes...when they are stretching, and I plant them in cell-packs and eventually, I put them in the cold windowsill as well. They seem to be alright that way, and I think that I have done that before, ages ago...
When we go to Folsom or Sacramento, I check out the garden dept. and see if they have any supplies that I need. Actually, I have what I need for now. Thanks for the tips...
I imagine I would have to research Vermiculite vs Perlite more to make any comments.
I DO know that Vermiculite DOES contain shiny, luminous flakes that are asbestos like. There is a bigger-particle Vermiculite sold, commercially, I think---that is almost like small popcorn. That may be safer than the flaky, small bits one. I still have a half of a bag from who knows how long ago! Just found it the other day in my Shop.
I could check out the ingredients...
You can always buy inexpensive aluminum trays or cookie sheets.
Anything that can hold water will do. Meat trays...cake trays...Deli trays---whatever.
Yes! having my seed set-up in my bedroom really makes me pay attention to it. Morning and night--as I turn on and off the lights--I always check everything out.
If it were in my basement or somewhere else--I might just forget about it. The old--"Out of sight--out of mind" applies here...
I am having chronic problems with my lungs and a persistent cough that will not go away. It took a Lung Specialist to zero in on Vermiculite. I had not read the label and was just having a good time mixing my combination with the wind blowing it back in my face. Look it up on the internet. You will never use it again. Good news is the specialist says it can be fixed and soon, about 6 months, I will be back to normal. There are many states that it cannot be sold.
I am shocked, Worms & Gita...California has the strictest rules in the country having to do with everything, but especially to do with anything agriculture and horticulture.
I was told that it was just expanded lava. I will have to look into it as well. I cannot imagine that CA would leave itself open to lawsuits in that way, especially with the state of our failing economy. YIKES!
What a great thread! You've all offered some excellent ideas here. I'm taking notes. :)
I've undertaken my first real attempt at seed starting this winter and so far I've had great success--no damping off and a tremendous germination rate. I'd be inclined to believe I have a natural green thumb...but I think my success was mostly due to the product I used, which has been great for a newbie like me.
I probably sound like an ad for Burpee's, but honestly I have to say this set-up just worked for me. Most of my seeds germinated in just a few days, even when the seed packets told me they would take longer. The pelleted seed starting mix is nice and fluffy and easy to work with, and the system has a mat that wicks up water from a tray beneath the seedlings. It requires added water once a week, so I never worry about my seedlings drying out. Ever. I appreciate any help I can get as someone new to seed starting.
The only fly in this ointment is that the starting kits (with 72 cells) cost $20 each. I know there are much cheaper ways to start seeds, such as inexpensive flats and starting mix. I guess I'll progress to those when I have more confidence. But right now I appreciate how these Burpee kits took all the guesswork out of seed starting for me and made it dead easy.
And no, I don't use the plastic cover that comes with the kit beyond the first few days. Once I see seedlings nosing out of the starting mix I remove it, since with this kit moisture retention isn't an issue.
I can't tell from the picture in the link you posted--BUT--in case those little peat pots have any kind of mesh around them--you need to slit it in a couple of places when you go to plant your seedlings out in the garden.
That thin mesh can inhibit roots from growing into the surrounding soil a bit.
Thanks for the heads up, Gita! Yes, I've used the pelleted mesh things in the past and I'm not fond of them. While this system does come with pelleted growing material that you "puff up" by adding hot water, there's no mesh, which is partly what sold me on it.
Here's a photo of some seedlings planted 3 weeks ago (each of them had germination time tables of 7 days or more, but all came up within 3 days). On the far right are tomatoes (cherry and Roma). Those are only at 10 days germination.
I admittedly planted the thunbergia way too early--look at 'em go! I do plan to transfer these to larger peat pots to give to family and friends and even sell a few at yard sales. How big they get this year before early May will determine when I start them next year.
I have this site saved--and I think it is time to post it again. It will help a lot--if you are not faamiliar what newly emerging seedlings actually look like...This would beoutside--where seeds come up on thir own--not inside--where you know what you have seeded...
This is NOT all-inclusive! But good enough!
This is WONDERFUL! Hello all! I am a newbie here in Southern CA - I had beautiful little beet sprouts all week and I went out this morning and most of them were wilted and gone! :( I realized reading this that I probably kept the lid on one day too long and also they are in direct sun - I will replant and keep them in the shade - and will also vent the lid when sprouts start...
I bought a deep plastic tub (for deep root room) at Home Depot and made holes in the bottom then I layed a thin (flexible) piece of rubberized plastic over the top. (drawer lining from Ikea) It's about 12" deep - I hope that works - I have been reading about container growing...
I have also planted lettuce and broccoli - all are doing well so far - I am going to vent the broccoli today though.
I am so happy to have found this site. I am just beginning my journey with veggies. Thanks for all the valuable info. You will probably be hearing from me again... ;)
Art33's question at the beginning of the thread was:
>> "If I make sure the mixture does not dry out, wouldn't it actually be better to not use the plastic cover?"
Yes, if you mist it often enough that the surface, or the seeds, stay constantly just a little moist, until they have emerged, the cover serves little purpose. But it saves you a lot of misting, and you can safely leave the soil less damp, because then it will never dry out overnight.
Less humidity means less chance of mold or fungus or plant-unfriendly bacteria building up on the soil surface. But using clean mix (called "sterile") Reduces that risk also. And removing the humidity dome as soon as any seedlings emerge also reduces the risk of "damping off" (seedling stems rotting right where they touch the soil surface).
Being able to keep the soil surface just SLIGHTLY moist, but not damp, and certainly not waterlogged, is really crucial. Seeds and roots neeed to breath oxygen, and water-logged soil has no air and almost no oxygen. Roots drowning and seed rotting are even worse than high humidity increasing the risk of fungus.
The soil (or rather soil-less seed starting mix) MUST have air channels, which means excess water MUST drain out.
Skilled gardeners seem able to water so lightly that they never get their mix water-logged, but I gave up on that. It seems I can't stop over-watering. I want to see water come out the bottom!
So I may start with expensive, peat-based store-bought seed-startinjg mix, but then I add a lot of screened pine bark mulch shreds or nuggets. Coarser than coarse Perlite, this assures fast drainage, so I can water until it comes out the bottom without drowning the roots and seeds. Since I'm cheap, and the fine bark shreds hold some water, I use lots more bark than mix. The bark nuggets or mulch can be as cheap as $4.10 for 2 cubic feet. I think Pro Mix or Fafards costs four times as much.
I agree with trc65 - it's ideal if your plastic cover has a slight misting, not big beads of water. If big beads collect, the soil is probably too damp. Let it vent so some water can escape.
When the surface dries out enough that air can enter it, the seeds will stop gasping for oxygen and drowning. Listen closely! Youm may be able to hear little seed voices squeaking "we're drowning in here!"
You can speed that up by setting the too-wet tray on top of a towel or cotton flannel. If the absorbant cloth touches the mix through the hole in the bottom, some of the excess water will wick down into the towel and air will return to all of the soil column.
Or, if you have the magic green thumb like trc65, somehow avoid over-watering seeds in the first place.
P.S. I have more seedling trays than I have humidity domes. But I have some 18" plastic food film ("Saran Wrap"). I drape that over a tray until part of the tray sprouts, then I remove the plastic film from that part. Or use dry cleaning bags or other clear film.
If I want to "tent" the film, I prop it up with plastic stakes cut from 1/2 or 1/3 the width of a venetion-blind slat, now called mini-blind slats. I also use those to label rows in the trays. I write on the white plastic slat with a mechanical pencil (0.7 mm lead or 0.9 mm).