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Green Soil in the Garden: Reasons and Resolutions

What Causes Green Soil in Garden

Should you start losing sleep over that slimy green mass creeping over your garden soil? Well, let’s just say – it’s not exactly time to summon the garden emergency squad!

When your garden soil decides to go green in a weird way, don’t panic. It’s a common spectacle in the gardening world, be it indoors or under the open skies.

The slimy green layer is usually algae that love warm, moist, and well-lit environments. Soil that receives excessive sunlight and water is prone to algae growth.

Greenhouses, with their high humidity levels, also encourage algae growth. Let’s explore why this happens and the ways to minimize the risk.

What causes green soil?

What causes green soil
Image by The Daily Garden

While algae can offer benefits by providing nutrients to the soil, excessive growth, especially in aquatic environments or on the soil surface, may compete with plants for resources and sunlight.

Most algae live in water, whether it’s freshwater or saltwater. Algae found in gardens prefer damp and humid places, like greenhouses or wet soil although they need light and water to grow.

Water and Humidity

Enclosed places like greenhouses and indoor areas become suitable for algae growth due to the higher moisture levels. This is because the chances of algae growth are higher when there’s little wind or airflow in or surrounding water-retaining materials.

Different types of algae prefer different pH levels, which can be found in freshwater, marine, and alkaline water conditions. Freshwater algae like a pH of about 7.0 while marine algae do well in slightly alkaline conditions around 8.2.

Of the 3, spirulina does best in somewhat alkaline environments, closer to a pH of 10. Just to compare how different algae are, most plants thrive in slightly acidic to neutral pH, which is around 6.0 to 6.8.

This soil pH range for most plants works best for freshwater algae, which is the most common type found in gardens.

Light and Temperature

Algae thrive in well-lit areas, such as in greenhouses as well as on potting soil when starting seeds indoors under grow lights. The best temperature for algae growth is between 68 and 86 F.


Algae need nutrients to grow which they can absorb and gobble up from the soil. A lot of times, soils with high nutrient content promote algae growth.

Spread and Reproduction

Smaller forms of algae multiply by splitting cells while larger ones rely on spores to reproduce. When conditions are right, the green algae can blanket the soil’s surface, blocking water access.

In some greenhouses, algae flourish on clear plastic walls due to abundant sunlight and minimal plant competition. If you’re not careful, these walls will become hotspots for algae when humidity increases.

Luckily, since algae have no roots, you can eliminate them just by spraying with a hose or scrubbing with soapy water and a sponge.


Algae doesn’t directly harm your plants because it’s not a plant parasite or disease. It can affect them indirectly in a couple of ways though. 

For one, algae competes with your plants for soil moisture and nutrients. Plus, over time, it forms a tough black layer or crust on the affected soil. 

This layer can make it difficult for water to reach the soil and your plant’s roots, even if you provide the soil with careful watering. You might even notice water pooling on top of the green or black algae before draining away without affecting the soil under the crust.

Two, if algae accumulates in your soil, it can retain excessive moisture, causing mold or disease issues for your plants. This can result in either dehydrated plants or roots rotting due to excess moisture.

How to Safely Get Rid of and Prevent Algae

How to Safely Get Rid of and Prevent Algae
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Managing algae in your garden soil can be tricky but you can remove it and stop it from coming back with the right approaches. Let’s look at some simple methods to deal with algae in the garden.

Proper Soil Drainage

Proper Soil Drainage
Image by Plantura Magazin
Ease of GrowthBeginner to Intermediate ●●○○○
Tools NeededShovel, compost, raised beds (if necessary)
Estimate CostsMinimal (compost and raised beds, if needed)

Moisture in the soil is one of the biggest reasons for algae growth, and drainage is a prime factor for moisture retention. If you notice this in your garden, then this could help you in getting rid of the green slime.

How to Have Proper Soil Drainage
1. Enhance the drainage. 
Add compost to your soil to enhance its structure and drainage. The larger, rough texture in the soil will allow more water to seep in without retaining too much moisture.
2. Raise garden beds if needed. 
If your garden gets too wet too often, use raised beds instead. The soil in raised beds drains water better than the soil in regular beds. 
Raised beds lift plants above soggy soil, stopping extra moisture and algae growth. Add some compost while you’re using raised beds.
3. Don’t overdo the mulch.
Use mulch moderately and distribute it evenly. Maintain a thin, even layer of mulch to retain soil moisture without encouraging algae.
Avoid piling the mulch near plant bases too. This is because mulching this way traps moisture and promotes algae growth. 
4. Stay away from peat moss, perlite, and vermiculite. 
These gardening materials hold a lot of water, making any growing medium with them stay damp and promote algae growth. 
Peat moss, perlite, and vermiculite can keep and trap too much moisture in your soil. If you have these materials in excess, they will create favorable conditions for algae to flourish.

Reduce Shade

Reduce Shade
Image by European Scientist
Ease of GrowthBeginner to Intermediate ●●○○○
Tools NeededPruning shears or loppers for trimming branches
Estimate CostsMinimal (pruning tools, if needed)

Algae needs some shade and just the tiniest bit of light to thrive. If you have green soil due to algae, think about reducing the shade and increasing the brightness.

How to Reduce Shade
Trim back trees and shrubs. 
Cut back branches of trees or shrubs that shade your garden soil to let more sunlight in to prevent algae growth. You can use pruning shears or loppers for this task.

Adjust Watering Habits

Adjust Watering Habits
Image by Housing
Ease of GrowthBeginner to Intermediate ●●○○○
Tools NeededSoaker hoses, drip irrigation system (optional)
Estimate CostsMinimal (soaker hoses, drip irrigation system, if desired)

Soil can get too moist from constant and even improper watering practices.

How to Adjust Watering Habits
1. Water early in the day. 
Water your garden in the morning, not at night. Morning watering lets the soil dry out during the day, making it harder for algae to grow. 
Just use a hose or watering can to water your plants’ base in the morning. Plus, watering early in the day makes your plants less vulnerable to leaf burns caused by water on the leaves exposed to the strong rays of the sun.
2. Try using soaker hoses.
Use soaker hoses or drip irrigation as an efficient and very cost-effective alternative to overhead watering. Soaker hoses send water to plant roots, cutting down on surface water. 
This saves water and lowers the risk of excess soil moisture, which can lead to algae growth. You can install soaker hoses or drip irrigation to keep your garden watered without promoting algae if you’d like.

Manage Fertilizer Application

Manage Fertilizer Application
Image by Ugaoo
Ease of GrowthBeginner to Intermediate ●●○○○
Tools NeededBalanced, slow-release fertilizers
Estimate CostsMinimal (cost of balanced fertilizers)

Excess nutrients contribute to quick algae growth. Here’s how to make sure you’re keeping the green soil controlled.

How to Manage Fertilizer Application
1. Use balanced fertilizers. 
Use balanced, slow-release fertilizers instead of high-nitrogen ones. Too much nitrogen boosts algae growth, especially when applied generously in damp places. 
Balanced fertilizers give your plants the necessary nutrients without causing imbalances that help algae thrive.
2. Follow product guidelines. 
Stick to the suggested application amounts mentioned on the fertilizer labels. Overdoing it with fertilizer can lead to nutrient runoff.
Fertilizer runoff creates nutrient-rich water that encourages algae. Plus, it can cause nearby water sources to be too full of nutrients, possibly encouraging algae growth there. 

Physical Removal

Physical Removal
Image by Coastal
Ease of GrowthBeginner to Intermediate ●●○○○
Tools NeededHand trowel, gardening gloves
Estimate CostsMinimal (gardening gloves if not already owned)

Sometimes, you just need to take off that layer of algae that’s gotten a bit too stubborn. This is one of the most effective yet time-consuming approaches.

How to Do Physical Removal
1. Scrape the algae. 
Use a hand trowel or a similar tool to scrape the soil’s surface where algae are visible. Don’t dig too deeply or roughly as doing so disrupts the nearby plant roots.
Wear gloves to protect your hands from possible accidents.
2. Dispose of algae.
Collect the algae and dispose of them correctly. Don’t leave the removed algae in your garden because they can rot and even promote more algae growth.
3. Monitor and repeat. 
Keep a close eye on your garden and do this whenever physical algae removal is needed. Spotting and getting rid of algae early stops it from spreading and causing more damage.

Use Algae Control Products

Use Algae Control Products
Image by Live Science
Ease of GrowthIntermediate to Advanced ●●●●○
Tools NeededCommercial algaecides (as a last resort)
Estimate CostsVariable (cost of commercial algaecides)

These products are pretty effective as they’re commercial algae killers. Use them carefully and only as a last resort.

How to Use Algae Control Products
1. Buy the right product. 
You can find them in gardening stores or online. Just make sure they’re trustworthy. 
When dealing with stubborn algae growth in your garden, use commercial algaecides as a last option. Try other preventive methods first.
2. Read and follow instructions carefully. 
If you choose to use algaecides, make sure to carefully read and follow the product instructions. Commercial algaecides can work well in managing algae, but it’s essential to use them sparingly and only when absolutely needed.

Prevent Algae Growth with Regular Maintenance

Prevent Algae Growth with Regular Maintenance
Image by The Conversation
Ease of GrowthBeginner to Intermediate ●●●●○
Tools NeededGarden gloves, mulch, moisture meter (optional), pruners, loppers, compost
Estimate CostsMinimal (mulch, moisture meter if needed)

When it comes to preventing algae growth, there’s nothing better than keeping a sharp eye out for them. Sometimes even the best gardeners can forget one or two items, so we’ve compiled a list of all the checks you need to make.

How to Prevent Algae Growth with Regular Maintenance
1. Control weeds. 
Keep your garden clear of weeds. Check for weeds often and remove them quickly.
2. Monitor soil moisture. 
Monitor soil moisture as these slimy green growths love watery conditions. You can use a moisture meter or touch the soil to check. 
Adjust your watering to prevent overwatering. Overwatering promotes excessive soil moisture and encourages algae growth.
3. Mulch mindfully. 
Apply mulch well, meaning spread it out evenly but not too thickly. Avoid making wet spots under the mulch, as it can cause algae to grow. 
4. Prune regularly.
Keep your garden plants and trees well-maintained through regular pruning. This helps reduce excessive shade, which, in turn, prevents algae growth.
Plus, pruning promotes better air circulation. With better air circulation, humidity levels drop and discourage the growth of algae.
5. Encourage healthy soils. 
Enhance your soil with compost and organic matter. Quality soil retains moisture effectively without becoming overly wet, which prevents algae growth and benefits your plant roots.

Use Quality Potting Mix for Starting Seedlings

Use Quality Potting Mix for Starting Seedlings
Image by Empress of Dirt
Ease of GrowthBeginner to Intermediate ●●●●○
Tools NeededPotting mix (store-bought), greenhouse covering (if necessary)
Estimate CostsMinimal (potting mix)

The use of potting soil has lower chances of containing algae spores compared to garden soil. Where and when possible, use quality potting soil, especially when growing seedlings or plants indoors.

How to Use Potting Mix To Start Seedlings
Purchase quality potting mix for starting seedlings.
Using a quality potting mix lowers the chance of introducing algae or spores from outside into your seedlings. You can find it at gardening centers or online.
When growing seedlings in a greenhouse, be careful. Wind-blown dust or dirt can bring in algae spores.
If you see algae on the soil, deal with it quickly. You can try drying the soil or covering it to stop the algae from spreading.

Use A Fan For Indoor Growing

Use A Fan For Indoor Growing
Image by Safer Brand
Ease of GrowthBeginner to Intermediate ●●●●○
Tools NeededFan, indoor grow room or greenhouse (if applicable)
Estimate CostsLow (purchase of a fan, if necessary)

Stagnant and humid air is one of the major factors for algae growth. Prevent that from happening by using fans to encourage better airflow.

How to Use A Fan For Indoor Growing
Install a fan or several fans.
When growing indoor plants, good airflow helps avoid algae growth. Algae likes still, damp conditions, so using a fan can help by drying the soil’s top layer and making it less suitable for algae. 
Just make sure that the fan is placed to move air across the soil and around the plants.

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