I had this problem this year with Miracle Grow Seed Starter. Last year, I started most of my seeds with my local nursery's brand of soil. I noticed this year that those seeds started with Miracle Grow ended up with moss/algae/green slime on them. The other seeds were less likely to have them. The difference was enough to convince me to not use Miracle Grow in the future for seed starting.
HMMMM- I DID use the Miracle Grow seed starting mix! I guess I wont do that agian!.
I have a flat of those jiffy pellet things- I think I will replant my leftover seeds in them.
and it is finally time for me to start my tomatoes and peppers!
Misting with very dilute hydrogen peroxide will deter the algae or moss.
I seem to get it when I use chamomile tea as the soil sterilizer.
When I sterilize with dilute (under 1%) peroxide---it does not happen.
I usually us about a .3% solution as sterilizer.
The problem is twofold first too much moisture and second the nutrients that are present in MG soil mix (and any soil amendment sold under MG brand). The same is true for chamomile tea, the nutrients in the tea will promote algae growth if other conditions are favorable.
Substituting misting for plastic domes may not help your problem as you will still be keeping the soil surface wet. Unless you have seeds that are surface sown and need light to germinate, water from the bottom only with no misting. Seeds need much less moisture than you think to germinate. Dryer is better.
A note of clarification: Chamomile tea and peroxide will not sterilize soil. At best they are disinfectants with some anti fungal properties. This may be a small difference in practice, but there are many soil pathogens, insects (eggs) and weed seeds that will not be affected by either chamomile or H2O2.
I like to make my seed-starting mix drain fast (and retain air spaces) by mixing commerical mix with screened pine bark from mulch.
I found that grit and coarse Perlite were not coarse enough to protect my seeds from my overwatering, if I used typical "peat powder" commerical seed-start mixes.
If the mulch passes through 1/2" hardware cloth, it is plenty small enough (unless you are starting small seeds on the surface, like Petunias or Lobelia.
I try to remove as many bark fines as I can, by starting with a good grade of "medium" mulch, $8/ 2 cubic feet, then screening with 1/4" hardware cloth to remove fines. The bark fines are a good amendment for outdoor clay soil, but I want chunks and shreds for seedling mix lightening - not powder.
Sometimes i try to use 1/2" hardware cloth to remove the fines by only using mulch that passes the 1/2" holes VERY QUICKLY. Like, after a regular horizontal screening with some rubbing, tilt the screening at a 30 de4gree angle and pour the shreds down the slope. What passes right through might be too fine for my purpose.
Even adding bark shreds only to the top layer of a flat or seed-start cell may dry the surface fast enough to prevent algae and mold (damping off). Plus, I like a little fan action to keep air moving.
If you're like me and found real bottom watering messy, or didn't have a good setup for bottom watering, consider "sort of bottom watering".
Lay down an absorbent cloth or capillary mat in the water-holding tray that your seed-start cells or pots sit on. I like cotton flannel or a towel, but I've used rayon batting, and I'm told that commercial capillary mats work great and last enough years to be as cheap as cotton flannel.
The seed cells or six-packs have to sit down ON the cloth, with bottom holes in contact, not be suspended above the cloth. There has to be a little contact between your seed-start mix and the cloth or mat.
It helps you water less often, by assuring that the water you add to the TRAY (not on top of the soil) is wicked equally to every cell or pot. If your tray has ridges, the cloth pulls water up from the bottoms of the ridges and supplies it to the soil equally, everywhere in the tray. The tray needs to be on a fairly level surface, but capillary action in the cloth will even overcome some tilt.
Donít add so much water at any one time that you have standing water visible. Let the mat and the soil absorb all of what you add. If the mat is clearly damp everywhere, water IS being provided to the soil.
The soil mix will wick enough water up to the seeds. It's only 2-3" at most, and capillary action takes care of it. The mat will probably stay somewhat moist for a day or two, but you donít want more water than that at any one time.
Don't spray or water the soil surface after the first day, when you planted the seeds.
Remind yourself that top watering drives out air. Drowning seeds or rootlets does suffocate them.
Seeds and roots are like people: they need lots of air freely available all the time, or they die quickly.
Trc55 is right: "Dryer is better" ... until the soil 1/4" down is totally dry.
(Maybe leave one cell in each row without seeds, so you can dig down into it to re-assure yourself.)
MAYBE, if you have no humidity dome or Saran Wrap, a LITTLE misting every 2nd or 3rd day won't hurt MUCH in a dry atmosphere with drafts or a fan. But the seeds only need a little water. With a fine soil mix, even a little more water than needed can drive out all the air, and you canít hear those seeds drowning and suffocating! They just never come up, or barely make it to surface, gasping and stressed and unable to resist damping off.
Instead, make yourself WAIT until the soil surface is visibly dry, by the bark changing color from dark brown/black to light brown/red. Even the mat should be fairly dry. If the mat is at all damp, the soil is also that damp.
Try to WAIT another 12-24 hours. The soil under the surface, in contact with the seeds, is probably still plenty moist, if the whole tray was watered uniformly 3-5 days ago. The absorbent cloth or capillary mat makes "sort of bottom watering" very uniform.
The rule I try to follow now is "just barely not DRY". When i aimed for "damp but not soggy", I always got "soggy". You have to trust that only the surface dried out. The seeds, 1/8" to 1/4" down, DO have enough water ... but do they have enough air?