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Mold on indoor seedlings

Staten Island, NY(Zone 6b)

Experiencing a bit of mold growth on some soil blocks I've made and have set up indoors. I made them using a homemade blocker (PVC Mold + Large Bolt, wood press, and a couple nuts) (the blocks are about 3" width x 3" depth). I wanted to start with bigger blocks so that I would not have to 'block-up' after some time. They are in a basement with temperature of 60°F and about 45% humiditiy, steady.

The soil blocks were made with: Peat, perlite, organic potting soil mix, organic lime, organic fertilizer + water

I was starting some kale. It's been 5 full days since I made the blocks and added the seeds. This is the first day I've noticed the mold growth.

Yesterday was the first day I watered them since they were blocked - they were not THAT dry, I suppose. I misted them thoroughly from above and lightly from the bottom with the trays they are resting in (aluminum catering trays). I am guessing I may have been over-watering.

I have not had exposed them to lights for what would equivocate daytime, but I have ballasts hung (Using T5 & T12 6500K Fluorescent bulbs). I was waiting until something germinated to do this.

I have not turned on the fan that I planned to use for air circulation for them yet.

If anyone can share some perspective on what my mistakes are and what potential solutions are, that would be much appreciated.

I'm thinking cold + wet are my enemies here and I'm trying to think ahead to whether or not the fluorescent bulbs being lit over the plants is going to make any difference temperature wise, really. I'm also wondering if having the fan on would make much of a difference. I also had the thought that I might able to fabricate a wall of aluminum foil surrounding the trays that the plants are in AND put on the lights.

Someone suggested that my starting mix might not have been sterile and that I should microwave it before using it. I really don't know about that one.

Below's pictures of the blocks + mold.

Thanks for any input. It is sincerely appreciated.

Thumbnail by jpofsunya Thumbnail by jpofsunya Thumbnail by jpofsunya
(Pam) Warren, CT(Zone 5b)

I always over -water as I use capillary matting in self-watering trays to keep the maintenance down. I start seeds in two locations, a chilly city window in the early winter months. When we have a really cold spell my window garden can get down to the low 60's. When we open the CT house around the end of March, I start seeds there, too. We leave the thermostat at 60 degrees during the week when we are not there. The first year trying seed growing at the house I had terrible losses from the combination of wet, cold, and low light. Thanks to the generous sharing of knowledge by so many great gardeners on DG, I have learned a few techniques that have made all the difference.

The second year I added a T-5 light and timer and a heat mat. Now I have several in both places. This year in the city for the first time I added a thermometer to regulate the temp.

I water with 10:1 mix of water and peroxide. i use this mix for everything- moistening the soil, regular watering, filling the trays. The extra oxygen molecule helps feed the plants and prevents pathogens from growing. I don't worry about sterilizing anything any more.

My last trick is to sprinkle cinnamon on top of the soil after sowing or potting up, which also works well as a fungicide, and prevents gnats.

I now have an excellent success rate. Pic one is the city garden this week.

Thumbnail by Pfg
(Pam) Warren, CT(Zone 5b)

Oops, it posted before I added the pics from the house.

Pics 1-3 are from 2011, my first good year.
Pic 4. Added a 2nd light in 2012
Pic 5. A mini- greenhouse, bought the previous year, used to harden off plants as the weather improves outside.

Thumbnail by Pfg Thumbnail by Pfg Thumbnail by Pfg Thumbnail by Pfg Thumbnail by Pfg
(Pam) Warren, CT(Zone 5b)

Soooo.... For your problem I would say turn on the lights, 16 hours a day, which will help with heat, and start using the peroxide/water mix, 10:1. I'm not strictly accurate... 1 quart, 32 oz, 3 oz peroxide. One gallon, 128 oz, 12 oz peroxide. But I always measure, be careful, stronger mixes are use as a weed killer!

And use cinnamon. I keep it in a salt shaker near the trays, and sprinkle it on whenever I smell something a little musty or if even a single gnat shows up. It doesn't bother the babies at all.

Good luck!


Thumbnail by Pfg
Florissant, MO

Many folks have experienced mold problems when starting seeds in any kind of peat product. Peat products just tend to stay too damp. The slightest over-watering can cause the problem you're having. Plastic containers seem to work much better! Even when using plastic containers the top of the soil can develop mold if kept too damp.

Also, starting with bigger blocks to keep from "blocking up" later was probably not the best idea. Until the seeds sprout and develop roots, there's nothing to absorb the moisture in even small blocks, much less the larger ones. One of the main reasons for starting with small containers and then transplanting to larger ones later is because it's hard to maintain consistent moisture in the larger containers when the plants are so small. Also, the act of transplanting seems to stimulate the development of the fibrous feeder roots.

Lastly, Pfg's suggestion of using a 10% peroxide solution is a good one. I've done that for years and it definitely works!


This message was edited Feb 12, 2013 6:57 PM

Florissant, MO

I forgot to mention that the mold should not hurt your plants. The mold is just a symptom of over-watering. It looks bad, may compete for the nutrients, and block a little oxygen from reaching the roots, but it won't directly effect your plants. Scrape it off, the best you can, and let your soil blocks dry out a little before watering again (using the 10% peroxide solution).


Everett, WA(Zone 8a)

A question - you said "organic potting soil mix". If it was potting SOIL, that's not so good for starting seeds. Too heavy, too much organic matter, way too much organic matter. The organic matter is going to attract, feed and encourage anything biological from bacteria to flies.

That might be what someone meant by "not sterile". Even if you autoclaved a soil mix for days, as soon as it cooled and dust fell on it, it would start to grow mold, yeast and you-name it.

No soil mix is "sterile" in the sense that a nurse or surgeon would use the term. Neither are soil-less mixes, after the bag has been open for 60 seconds! But clean soil-LESS mixes probably have 10,000 to a million times fewer bacteria, mold, fungus and spores than real soil. And (I guess) 100 to 1,000 times fewer living things than fumigated, steamed and irradiated soil mixes!

I think you'll have much better luck making cubes for indoors with a soilless mix, like ProMix HP or ProMix BX or Sunshine #4 ...

Sunshine #4 gets more air into the mix, and retains less water, because it has extra Perlite added.

The following is probably irrelevant to making cubes ... bark chunks probably don't bind well.

A cheaper way to get the same improvement is to buy "whatever" clean soilless potting mix, then mix it with 25% to 75% screened bark nuggets or chips. Or fibers, if they are not TOO fine., Avoid bark powder, fines and fine fibers - peat mixes already have too much "fine stuff".

If you can screen out anything that passes through 1/8" mesh, what's left is fairly coarse and 15-30% is probably enough to improve something's drainage & aeration.

Or, since you're just using it as a coarse amendment and not for the "wicking" component, you could even discard bark that passed TOO easily through 1/4" mesh

But, if you are starting with a mix like Jiffy-Mix (powdered peat pudding mix), you might consider 90% bark and 10% peat!

Staten Island, NY(Zone 6b)

Thank you all for the time and input, everyone.

Perhaps I should mention the mix for the soilblock proportionally:
3 Buckets Peat
1/2 C Lime
2 Buckets Perlite
3 Cups Fertilizer
*1 Bucket Soil
*2 Buckets Compost

*This is where I used the potting soil mix - to replace soil + compost called for in the recipe. The potting soil mix is mainly composted bark, with peatmoss and pasteurized poultry litter.

So, I do have a pretty good portion of the potting mix being peat, RickCorey_WA. It seems the Promix and Sunshine products are peat thats been inocullated.

With what your saying about bark - could I add more perlite to the mix, instead, for better drainage.

I am wondering if starting with larger blocks is not healthy, as art33 suggested.

I have about $90 invested in the starting materials, as is. Looks like the closest retailer on the Promix is about a $10 toll and another $50 or so? I have a rhizobium inoculant I purchased in 2009 - forgive the ignorance but would this be beneficial at all to add to a block mix? If this is too old to be useful, what about something fresh?

My compost pile was redone this winter and is not broken down. and I cannot source a quality organic compost until February 28th, locally.

I'm pondering the hydrogen peroxide and whether or not this would have a negative impact ecologically with respect to the soil, once the blocks are outplanted.

Thanks again.

(Pam) Warren, CT(Zone 5b)

Peroxide is water with an extra oxygen molecule. When exposed to air, the extra molecule breaks away and what remains is water. That's why if you don't keep it tightly covered it goes flat. H2O2 is the chemical formula. Water is H2O. So no lasting ecological effect.

Florissant, MO

Yes, as Pam has mentioned, hydrogen peroxide (H2O2) is just a combination of hydrogen and oxygen, it won't harm our environment a bit. Also, when talking about using a 10% peroxide solution, I'm referring to mixing one part of the standard strength peroxide with nine parts of water. The standard strength of peroxide, that you buy at a pharmacy such as Walgreens, is 3% strength. So when that is mixed with nine parts of water, the actual strength is reduced to only 3/10 of one percent.

Below is a link to a page that explains about some of the benefits of using hydrogen peroxide in the garden.

Hope this is helpful,


Everett, WA(Zone 8a)

>> could I add more perlite to the mix, instead, for better drainage

Yes, and that is what most people do. I just like bark.

>> I have a rhizobium inoculant I purchased in 2009

I think that is the nitrogen-fixing innoculent for peas and beans. Very helpful if you are growing legumes. But I think it's not the same as endo-mycorrhiza such as most non-woody plants need to live in symbiosis with their root hairs.

At 3-4 years old, I think that legume innoculent has lost most of its viability.

But I've read that some endo-mycorrhiza products contain spores (longer lasting), while others only have fragments of fungal hyphae (short shelf-life).

IF legume innoculent products work the same way, and IF yours had mostly spores, MAYBE it would still have some value after 3 years. For peas and beans.

Staten Island, NY(Zone 6b)

Thank you all, again. I appreciate all the input re ecology and chemistry and such. I was aware of h202 chemistry but molecule can also make something lethal or not....

The kale seedlings sprouted today, so I've turned on the lights.

I'm wondering if I can avoid this issue in the future if I:
1) start with smaller blocks
2) add more vermiculite
3) water less
4) have fan blowing

I'm also interested if anyone else perceives this to be a potential threat to the seedlings, etc. I appreciate your commentary towards this not being too much of an issue from your perspective, art33.

I did find this, speaking about the endo-mycorrhiza. With all this, I'm sure it's quite different from what I was thinking of - thanks for helping to clarify this.

"If you have any interest in gardening or farming, there is another player in addition to the plants and soil that you should know about: mycorrhizal fungi. This type of fungus forms a symbiotic relationship with approximately 90% of plants! The fungi colonize the roots of the plant and then extend their hyphae far into the soil, bringing nutrients and water that would otherwise be out of reach to its host. In return, the plant provides the fungus with carbohydrates.

There are two types of mycorrhizal fungi. Shown in the picture below, endomycorrhizal fungi associate with many agricultural crops and ectomycorrhizal fungi mostly associate with trees. Endomycorrhizal fungi penetrate the plant roots cells, while ectomycorrhizal form a layer around the root. Ectomycorrhizal are classified by producing mushrooms above ground, which endomycorrhizal do not do. Some popular edible mushrooms, such as chanterelles and truffles, are ectomycorrhizal. The entire underground structure of the fungus is called the hyphal network.

The plant receives many benefits from the fungus. The most obvious is the increase of water and nutrients. This is especially important for immobile nutrients, such as phosphorus, that do not move with water so the plant would have to be in direct contact with to access. Disease rates are lower in plants with a healthy symbiosis because the fungus is already taking the carbohydrates that pathogens would want. Mycorrhizal fungi connect all plants in an ecosystem and the plants therefore take part in a nutrient exchange: plants with excess nutrients can pass nutrients to those in need. This is especially helpful with seedling establishment. The interconnected web also picks up nutrients from dying roots so plants can access these nutrients right away, instead of waiting for them to decompose, and it eliminates any chance of nutrients leaching away into the soil. The extended hyphal network of the fungus produces a sticky substance called glomalin, which keeps soil in place and prevents erosion.

All of these benefits result in more productive plants. Why wouldn’t more farmers be all about mycorrhizal fungi? Unfortunately, I don’t think most farmers even know about this symbiotic relationship and many of modern agriculture’s practices create a fungus that is actually detrimental to the plant.

Tilling and turning over the soil every year destroys the hyphal network so that the fungus must rebuild itself each year. This gives advantage to only the fastest growing fungus, which is not the most beneficial. Another harmful practice is chemical fertilizers. Since plants with fertilizer always have enough nutrients and don’t need to rely on the fungus, they give off fewer carbohydrates. The fungus must then become more aggressive in order to obtain the same amount of carbohydrates, which harm the plant. A study was done on fertilizers and fungi that compared three treatments: grass inoculated with fungi with fertilizer, grass inoculated with fungi without fertilizer, and grass without fungi. The grass with fungi in fertilizer was actually less productive than the one without fungi, proving that fertilizer makes it detrimental! Finally, monocultures also lead to aggressive fungi because the fungus can learn to adapt to the plants. In a more diverse garden, the fungus cannot do this. Look at that – more reasons to avoid these practices we already knew were harmful!

Although mycorrhizal fungi naturally associate with plants the species in your yard might not be the most beneficial, for all of the reasons listed above. To solve this problem, there are commercial inoculants for sale that you can use to introduce the best species to your garden. You should choose powder products. These are most effective if added to transplant roots, but you can also mix them directly with seeds and then plant those. A helpful website can be found here:"

Many thanks.

This message was edited Feb 13, 2013 11:56 PM

Ottawa, KS(Zone 5b)

Hi JP,

"I'm wondering if I can avoid this issue in the future if I:
1) start with smaller blocks
2) add more vermiculite
3) water less
4) have fan blowing

Sunshine usually inhibits mold growth. However, we haven't established that the mold did any actual harm, have we?


Everett, WA(Zone 8a)

More vermiculite will hold even more water, which seems undesirable.

Very coarse vermiculite might hold more drainage channels open, until it crumbles!

Coarse Perlite or very coarse grit will hold LESS water and create better drainage and better aeration. Ideal grit size might be around 1/8" or 3 mm (1/10 inch to 3/16" or 2.5 mm to 3.5 mm). Or you could start with a mix like ProMix HP (high porosity) that drains fatser, holds a little less water, and promotes better air diffusion.

Both of these sound good, especially watering less. Over-watering is my downfall!

3) water less
4) have fan blowing

I agree that the endomycorrhizal fungi help roots ... and might even compete with undesirable fungi and tend to displace them. Maybe.

>> All of these benefits result in more productive plants. Why wouldn’t more farmers be all about mycorrhizal fungi?

Speculating, two reasons. They are somewhat expensive, and they are already present in any healthy soil. .

BTW, high quality profession seed-starting soilless mixes do have small amounts of endomycorrhizal fungi added (like $38 per bale).

Also, any soil that has had any vegetables grown in it before, and is at all healthy with a few % of organic matter supporting microbes, will have plenty of endomycorrhizal fungal hyphae, spores, and fragments of small roots containing more of the fungi. Wiping therm out would probably require high-pressure steam sterilization!

The following is not intended to be a rant. It's just long and opinionated. YMMV, I may be wrong, and I'm not trying to preach. But I saw that it did sound like I was up on a soap box after I typed it!

I question whether tilling destroys all fungal hyphae. It might reduce their numbers from thousands per cubic inch down to hundreds per cubic inch. It is very hard to eliminate 99% of any microbe.

One biologist offered a theory that "everything is everywhere", and whether you had any one variety of microbe in excess, in sufficient numbers, or not enough to detect, depended on conditions that encourage them or discourage them, and what competes with them. But very seldom is any microbe totally and 100% absent.

I think that excess phosphate discourages the EMR fungi, I'm not sure. That is another reason not to OVERfertilize. (EMR is easier to type!)

I agree that any practice that consistently discourages beneficial fungi and encourages useless or harmful ones decreases the numbers of desirable ones. But is it even possible to reduce the number of EMR fungi to the point where they could not inoculate root hairs and then spread throughout each root system?

Soil management MUST allow EMR fungi to persist enough to 'infect' new roots. And encourage them enough that they aren't struggling hard to come back from "very rare" every year. But you can get that (I believe) simply by avoiding large excess of phosphate and supplying moderate amounts of organic matter to the soil. (And growing anything with roots that support EMR fungi.)

There may be other reasons for the organic system to preach ZERO chemical fertilizer and HUGE amounts of organic compost every year. But that full-bore method is not [u]required[/u] to keep the EMR alive in sufficient numbers to colonize root systems in reasonably healthy soil.

That's just my opinion.

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