Tiny cultivation

Stirring up the green moss

Tending seedlings on this tiny scale is pretty much literally fieldwork in miniature, especially with the pesky GREEN MOSS. The seedlings have to be watered, of course. And with the green moss, they have to be weeded as well. I use something pointy to stir up the surface of the peat-perlite seedling mix…

It comes back quickly, in a day or two, whenever the surface is wet.

I’m not even sure it’s moss, could be algae? Mold? Lichen?! I haven’t been able to ID it for sure, but I’ve often seen it called…green moss.

Green moss before and after

Not too appetizing to look at, the green moss has been quite harmless. At the old farm, I wondered if it came from the well, but here, we’ve been using filtered water and there it still is. It could be from the peat.

In any case, no worries, just a quick scuffling once in a while to keep it from sealing off the surface while the seedlings are small, which it looks like it would eventually do. I suppose I could find some clever, NATURAL way to kill it off, but I really don’t mind it. Once the mix dries out, the green moss is gone. At least, it disappears…

22 thoughts on “Tiny cultivation”

  1. I’ve noticed that in my seedlings a couple times too, but have been afraid to “weed” for fear of disrupting the seeds.  Good to know that it is harmless, whatever it is, and the seeds are fine.  When you scuff them over, what tool do you use, and how do you keep from disrupting the seedlings?

  2. The mosses come from airborne spores–they are everywhere and will land and grow whenever a suitable substrate/environment  is available.  Its amazing how many bacteria, fungi, moss, etc. spores are floating around and how many we inhale with every breath!  Here’s more on the moss family (Bryophytes):


    You might try watering a little less, if that’s possible.

  3. Is this green crap not what causes “damping off” and the death of seedlings?  If so, looks like you’ve come up with a way to keep it from actually killing them.   I’m going to be trying kelp extract mixed with cider vinegar in my next round as that is purported to be a preventative.

  4. Damping off is caused by fungus, and this seems to be a green plant.  In my seed beds I believe it is moss and possibly algae.


    I suppose that the increased moisture from a mossy layer could provide an environment conducive to fungi that cause damping off.

    My main question about the moss on seed beds would be whether it ties up nutrients that the seeds might benefit from.  The potential for sealing the soil seems like it could be beneficial when seedlings are larger, as it could lessen evaporation and decrease watering needs. 

    Mike seems to think it is a net loss.  I would like to hear more about his reasoning, and any other opinions.

  5. I looked at these pics a couple of days ago and thought to myself, “I’ve never had that!” But lo and behold, when I looked at my trays this morning, some were green on top.

    This is my second year starting seeds. Last year I used sterile seed-starting mix for everything because I was so worried about damping off. This year I relaxed a little and mixed compost into the mix for the onions. I was a little paranoid about disease with the broccoli, so I kept their soil mix sterile. The onion trays are getting green on top. The broccoli trays are not. Interesting.

  6. Yup I have been having the green fungus/moss on the soil surface of my seedlings this year.  If you used pro-mix like I did for germinating seedlings this year then its understandable. Pro-mix is made up partially of peat moss  so it makes sense that we would find fungus or moss growing on the surface.  Over the past two years I have found other kinds of fungus growing from my use of pro-mix as well. While the spores are in the air, I am positive that most of the spores are to be found in the pro-mix itself.  Which moss or fungus grows depends on the batch of pro-mix you bought as well as temperature, light and humidity levels in the growing environment. If you want to have a truly sterile mix then heat small batches of your pro-mix for about half an hour in the oven at around 250 celcius, this will kill off most spores and fungus gnat eggs (several other kinds of eggs and larvae are also to be found in the pro-mix unfortunatley).

    Kind Regards
    Tamas Dombi
    Kind Organics

  7. I’ll go with Steve’s airborne spores. This year, I only washed the trays with soap and water, but in other years, I’ve done a pretty intense spring cleaning, including a soak in 1:10 bleach (5% sodium hypochlorite):water bath, and the green moss still came on.

    It really doesn’t bother me, since it doesn’t SEEM to be doing anything bad.  I stir it up just in case.

  8. I used sunshine organic mix this year with no moss or green stuff.  Tamas – which pro mix is ok for certified organic? I couldn’t find one here in BC.
    I am looking into washing with hydrogen peroxide as I think it more benign than bleach.

  9. While I agree that bryophyte spores are floating around in the air, I think that the main source for the green moss is, as Taras says, the potting mix itself. When I start seed indoors I cover the trays with clear plastic, which would prevent most airborne spores from contaminating the soil, yet they still get the moss quite quickly.
    Similarly to Taras, I now microwave any soil used for seeding or cuttings for 10 minutes, then let it cool down (covered). The soil should be wet going in, and steaming hot when it comes out of the microwave. I have had cuttings under plastic in this sterilized soil for months without any algae, moss, fungal, mold, or bacterial growth on the soil.

  10. I have had a good garden every year and never had moss, this year my garden is drowning in water from all the rain and the top of the garden is covered in green moss.  I thought it might be from all the rain.  It looks like a carpet of green today.  How do I get rid of it? When it first started, I raked the top off being careful around the plants, but now it came back.  Do I need to keep raking it off?  The next 2 days are expecting no rain, a blessing.  I don’t want to lose the garden, so much was planted.  What should I do?  Help!

  11. Mary: I’m not sure what you have in your garden, but in case you don’t get any other answers, I say, don’t worry. Let it dry out, scuffle it (break up the surface with a 3-tine cultivator or something similar) or rake it a bit, and I’d bet it’ll be fine!

  12. Hi there,
    I’m a grad student doing some research on plant disease resistance.  A big part of that is just growing bajillions of little cucumbers in our greenhouse, and troubleshooting all the various problems that come up.  I’ve noticed the green stuff on potting mix before but have never heard of it being a problem.  I’m going to go out on a big, cocky limb here and say that if after 2 years of grad school in agriculture I haven’t heard of it being a problem, it’s probably not a problem!
    I think it’s just a little layer of algae that’s growing because it’s wet and there are nutrients to be had- the plant roots aren’t going to be growing on the very surface anyway, so the algae isn’t eating anything the plants would be getting otherwise.  Because it’s green, that indicates that whatever it is makes its own food via photosynthesis, so it won’t be parasitizing your plants like a fungus would.
    Damping-off is caused by Pythium and Rhizoctonia, which also like wet conditions (as anybody who’s raised seedlings in a greenhouse has noticed!).  Interestingly, compost that’s gone through a good hot phase and then aged will inhibit these and other pathogens.  It depends on the compost, the type of plants, and the strain of pathogen, but you can sometimes get some really good disease reduction with 1-part-compost-to-4-parts-potting mix.  If you want references I can email them to you.

  13. We have the green stuff, we take care of it by sprinkling about an eighth of an inch of potting soil on top of the seedlings, then brushing the dirt off of the plants if we need to.  I have found that the seedlings grow better if I do this.  The green stuff grows back, but it is delayed enough to give the plants time to size up before transplanting.  If left alone it will form a crust that can keep the soil from getting the water that it needs, and keep air from getting to the roots.  I also think that fungus gnats will grow on it, and then they will eat the roots of plants.

  14. It’s algae.  “Scuffing” it up doesn’t hurt it or slow it down…in fact, it very well may be helping it grow by inoculating areas that weren’t already growing it.  That’s ok, though, because it’s not harmful.
    However, scuffing seems a good practice, maybe once a week, to keep it from getting thick and making watering uneven.

  15. Scuffing does work, especially if you do it as soon as it is noticeable.  Dry weather helps to keep it from returning.  Over watering or wet weather will help it return.  It has not bothered my plants just my sensibilites.

  16. Its horrible!!!!!!!!I bring my plants into science class and there always like whats that green stuff.Ive been trying to get rid of it by doing rather childish tactics but it doesn’t work.I cant let the soil dry up because then my carnivorous plants will die.It cant be looking for nutrients because theres none in my soil

  17. It is a problem with germination. The seeds will rot and seedlings cannot grow with the moss smothering them and then the gnats come and feed. Germination rates are drastically reduced. I, too, scrape off as gently as possible without disturbing the seed area and remove the infected top 1/8-1/4 soil, (PRO-MIX). Top dress with dry soil and water sparingly as needed. Air circulation is critical to reduce humidity levels/stagnant air. Being diligent is a must! I then transplant as soon as plants are large enough to do so. It is ALOT of extra work in a commercial greenhouse but it’s better than having to start over. Good luck!

    • Everything in moderation, of course. Green moss is extremely beneficial to keeping your plants moist and hydrated, it’s definitely not harmful. However, if you have it, you need to make sure you’re not over-watering. The whole benefit of green moss is that you don’t need to water as often or as much. As Kimbra says, it IS a lot of extra work in a commercial greenhouse to keep the balance between the moss and air circulation, but it’s worth it in order to keep all those plants thriving while keeping your costs down. If you’re truly worried I’d suggest using Oregon green moss from Oregon Evergreen & Willamette Evergreen (willametteevergreen.com) – great quality, no bad spores, and they sell wholesale to commercial greenhouses.


Leave a Comment