Why Is My Garden Soil Turning Green? (Plus Ways To Stop It)

If you see the soil or seed starting mix turning green in your garden or greenhouse, you are not alone.  Many gardeners have this same problem, either on their garden soil or indoors when starting seeds.  I did some research to find out what causes this problem, and what you can do to solve it.

So, why is your garden soil turning green?  A green layer on top of soil is often caused by algae, which thrives in warm, wet, bright environments.  A garden with soil that stays wet and gets plenty of sun is more likely to see algae growth.  Algae are also notorious for taking up residence inside of greenhouses.

Luckily, there are ways to get rid of algae and prevent it in the future without killing your plants in the process.  Let’s take a look at what algae are and where it thrives, and how you can control it going forward.

Why Is My Garden Soil Turning Green?

Most likely, a green layer growing on top of your soil is caused by algae.  This layer is often slimy and slippery at first, and can become black as it dries out.  So what is algae, anyway?

What Is Algae?

Algae is a broad term for an entire group of organisms which share some similarities with plants.  The group of algae includes single-celled micro-algae all the way up to giant kelp and seaweeds.

Kelp is just one of the many types of algae.

Just like plants, algae use photosynthesis to turn light into energy, and thus they contain chlorophyll.  This gives most algae a green color, although there are also brown and red algae.

Although algae are similar to plants, they do not have roots, stems, or leaves as plants do.

For more information, check out this article on algae from Wikipedia.

Where Does Algae Thrive?

Most algae are aquatic, meaning that they live in either freshwater or saltwater.  The algae you see in your garden will thrive in moist and humid environments, such as in a greenhouse or on wet soil.

algea in water
Algae is aquatic, and so it grows readily with light and water.

If there is little or no wind or air currents, then the environment will tend to stay moist.  This explains why you will see algae more often in a greenhouse or indoors.

Algae have the potential to thrive in anything that holds a lot of water.  This includes peat moss (sphagnum), perlite, vermiculite, and other moisture-absorbing materials that are often included in potting soil.  This is why you will sometimes see the perlite in potting mix turning green after over watering your houseplants!

Algae grow best in areas with plenty of light, which is why they tend to do well in greenhouses.  This is also why many gardeners see algae (or “green mold”) on seedling soil when starting plants indoors under grow lights.  Of course, algae are perfectly happy to grow on your garden soil if it stays wet and gets plenty of sunlight.

The optimal temperature for algae to grow is 68 to 86 degrees Fahrenheit (20 to 30 degrees Celsius).  If your greenhouse stays this warm overnight, then you just might be sending algae an invitation to set up a colony.

For more information, check out this article from Science Direct on the effect of temperature and light on algae.

The ideal pH for algae varies, depending on the type.  Marine (ocean) algae usually like a pH of around 8.2, while freshwater strains prefer a pH around 7.0 (neutral).  Spirulina (sometimes eaten by humans!) prefers a pH closer to 10.

For more information, check out this article on algae pH from Algae Research and Supply.

Unfortunately, the ideal pH for most plants is 6.0 to 6.8 (slightly acidic to neutral), so freshwater bacteria will feel right at home in your garden or greenhouse.  You probably won’t be able to alter pH to kill algae without also hurting some of your plants.

How Does Algae Spread?

Small algae reproduce by cell division, but some larger algae use spores to reproduce.  Algae can spread over the entire top layer of soil on a tray used to start seedlings.

For more information, check out this article about algae reproduction from Brittanica.com.

Algae are also notorious for growing on the clear plastic walls of some greenhouses, where they can get plenty of sunlight and grow without competition from other plants.

The walls of this greenhouse are a prime spot for algae to grow if the moisture level gets high enough.

However, since algae have no roots, it is easy enough to wash off with a strong spray from a hose.  You can also scrub it off with a sponge and soapy water.

Will Algae Kill My Plants?

Algae will not work directly to kill your plants, since it is not a parasite or a disease to your plants.  However, algae can indirectly harm or kill your plants in a couple of ways.

First of all, algae can compete with your plants for moisture and nutrients in the soil.  Over time, algae can form a hard crust on the soil, which makes it difficult for water to penetrate to the soil and to a plant’s roots.

Second, if algae form a thick layer on your soil, it can retain too much moisture, leading to mold or disease for your plants.

How To Get Rid Of Algae

Although algae do not always spell death for your plants, you may still want to get rid of it, just to be safe.  You might also want to remove algae for aesthetic purposes.  Here are some ways to get rid of algae if you already have it in your garden or greenhouse.

Dry Out The Algae

Since algae thrives in moist conditions, the best course of action to get rid of algae is to dry it out a bit.  One way to do this is to improve soil drainage.

If your garden soil drains poorly, it is more likely to stay wetter for a longer time, inviting algae growth.  You can improve soil drainage by adding compost, which provides organic material for the soil.  For more information, check out my article on improving soil drainage.

cracked clay soil
Clay soil drains poorly and may lead to algae growth on the surface of the soil.

There may be areas of your garden that stay wet for a long time after a rainstorm.  If so, consider taking steps to improve drainage by digging trenches and installing pipes to divert water away from these problem areas.

Also, get into the habit of watering your garden in the morning, not the evening.  When you water too late in the day, the soil stays wet overnight.  This increases the chances of algae, mold, and moss growing in your garden.

It is also worth considering the possibility that you are over watering your plants, which is keeping soil wet and allowing algae to grow. For more information, check out my article on over watering your plants.

If you are starting seeds indoors to prepare for the spring, consider changing your watering method.  Instead of watering from overhead, water seed trays from the bottom instead.  This allows the surface of the soil to dry out, preventing algae growth.

To water from below, all you need are some holes in the bottom of your seed tray.  Simply submerge the tray into water deep enough to provide water to the soil, but not deep enough to cover the top of the soil.

If you need to, you can poke holes in the bottom of your seed tray using a metal point (such as a shish kabob skewer).  Just be careful not to poke yourself!

Decrease Light Levels

Algae have been called “nature’s solar panels”, because they derive energy from sunlight.  If you want to inhibit algae growth, just decrease the light levels wherever you find algae growing.

algae on rock
If you go to the beach, you may see some algae on a rock, like this one.

This may be easier said than done, since your other plants may also suffer from a lack of sunlight.  However, if your grow lights are on for 16 hours a day to start your seeds, you may be able to tone down the light a bit to discourage algae growth.

For more information, check out this article on algae from Michigan State University.

How To Prevent Algae

Once you get rid of algae on your garden soil or in your greenhouse, you will want to take steps to avoid it in the future.  Here are some ways to prevent algae growth.

Use Potting Mix To Start Seedlings

If you are going to start plants from seed indoors, be sure to use potting mix from the store, instead of compost or garden soil (“dirt”) from your yard.  Otherwise, you may bring in algae or spores from outside.

You need to be especially careful when growing in a greenhouse, since algae spores on dust or dirt can be blown in by the wind.

For more information, check out this article from ProMix on controlling algae on growing media.

If you have had problems with algae in the past, it may make sense to use a humidity dome to isolate your seedlings and keep algae out. For more information, check out my article on humidity domes.

Avoid Peat Moss, Perlite, and Vermiculite

As mentioned earlier, these materials retain lots of water.  This means that any growing medium that contains peat moss, perlite, or vermiculite will stay moist and will encourage the growth of algae.

For more information, check out my article on perlite versus vermiculite.

Perlite and vermiculite, along with peat moss, may retain too much moisture in your soil, which can allow algae to thrive.

Avoid Over Watering Your Plants

Make sure to let the top of the soil dry out before watering your plants.  Otherwise, a top layer of soil that is constantly moist is inviting algae to grow.

Use A Fan For Growing Indoors

Air flow dries out top of soil to prevent algae growth, so consider using a fan to keep algae at bay if you are starting seeds indoors.

What Is the Difference Between Algae And Moss?

Algae form a thin layer on top of soil and grow in warm, moist, bright conditions.

moss on statue
Moss grows thicker than algae and often feels dry.

Moss, on the other hand, forms a thick mat (often called a carpet) on soil and grows in moist, dark conditions.  For more information, check out my article on moss.


By now, you should have a much better idea of what algae is, where it thrives, and how to control it in your garden or greenhouse.

I hope you found this article helpful – if so, please share it with someone else who can use the information.  If you have any questions or advice about algae control, please leave a comment below.


Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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