Q/A #136 - Algae on Soil Surface

July 28, 2023

Q/A #136 - Algae on Soil Surface

This concerns green slime algae on soil surfaces. I have this growing on soil mix (peat and pearlite) on seedlings I am starting. The pots are sitting in trays of water under grow lights. The soil is kept moist from tray watering. My question is how do you control this type of algae? I've read some articles on sprinkling cinnamon for control as algicides can damage plant growth. If you have this problem of slime algae, how do you control? The shiny surfaces shown in the photo is the algae.
(Submitted in July 2022.)


Algae slime mold is an ongoing problem for anyone growing carnivorous plants indoors. Spores of this type of algae is ubiquitous. We occasionally see it in a few pots in our tropical greenhouse. Fortunately, it's not at all detrimental to the plant. It's mostly unsightly, and we don't like seeing it either. While there's no magic bullet solution to this, but some things can help. 

The first would be to give the peat moss a soak and rinse before using it. It's a hassle, but it removes excess nutrients and spores that may already be in the peat. Top-watering some helps too, but hard on trays of small pots like this unless you have misters. 

If we see slime mold happening at our nursery, we scrape off the top layer and replenish it with fresh soil. We may even repot the plant. 

I know that cinnamon oil can be harmful to plant growth if used in high concentrations. However, we've successfully used neem oil and peppermint oil to get rid of powdery mildew on Sarracenia. We've just never tried it on slime mold, but it's worth a shot. If you try it, use a tablespoon of neem and a tablespoon of peppermint oil per gallon of water. Spray on the soil surface. You may need to reapply 2 more times several days apart. 

The other option is to repot all of your plants in fresh soil. 

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