Starting plants from seed is an excellent and budget-friendly way to create an array of plants in your garden. With indoor seed starting, all you need to do is gather light, seeds, soil, water, and food, just like when you grow plants outside. If you're starting to grow your seeds indoors, it's common to find a sudden plethora of green algae growing on the surface of your seed starting mix. This happens frequently when using peat-based mixes, which most gardeners use for seed starting. The good news is that it's not likely to harm seedlings, and it's fairly simple to prevent.
Removing Green Algae on Seed Starting Mix
To clear up any green algae growing on your seed starting mix, lightly cultivate the surface of the soil with a small tool, such as a chopstick or pencil. This simply breaks up the layer of algae. The algae itself won't hurt your seedlings, but it could cause problems if you allow it to keep growing. For example, algae growth can get very thick over time and start to hold too much moisture near the seedling's stem. This can cause issues like rotting.
Green Algae Growth
Green algae is a large and informal group of algae that contains chlorophyll. These eukaryotic organisms live primarily in freshwater and come about in many forms including unicellular flagellates.
When the site is muggy, algae growth is bound to grow on your seed starting mix. You'll know for sure when you see a bloom of pink, green, or brown sticky material moving across the surface of the soil. While it won't kill the seed immediately, it can cause issues with your soil's nutrients and water.
Causes and Prevention
Over 7,000 species of green algae are found in freshwater, saltwater, and damp spaces. The cause of algae growth comes from soil that stays too moist and has a lack of air circulation.
One way to prevent the growth of algae is to switch up your watering technique. For instance, if you've been watering from the top, you can try watering from the bottom instead. Simply fill the tray that your seedlings are in with about an inch of water. The water that isn't absorbed, after about an hour, should be poured out so you don't end up rotting your seedlings.
To help with air circulation, keep an oscillating fan on low near your seed starting racks. This will keep your plants from staying too damp and will encourage stronger growth.
Choosing a Seed Mix
Despite the algae growth commonality, peat moss mixes are an ideal base for many soilless mixes due to its hold on water and air. This light and fibrous mix can hold 15 to 20 times its weight in water alone. You can also consider a mix like composted pine bark or coir, which are renewable organic materials, unlike peat moss.
Bubel, Nancy, and Jean M. A. Nick. The New Seed-Starters Handbook. Rodale, 2018
Clarke, Ethne. Seed Starts & Smarts: An Organic Gardener's Guide to the Fundamentals of Growing Plants from Seed. Rodale, 2012