Algae in a Kratky Kit: Causes, Fixes and Prevention

Algae is the most common problem in a Kratky kit, and the easiest to prevent. It needs two things: nutrient solution and light. A Kratky reservoir always has the first, so the job is to take away the second.

What algae looks like in a kit

• A green or brown film on the inside walls of the container.
• Green-tinted water, sometimes with floating strands.
• A slimy green coating on the net pot, the grow media or the roots near the top.
• A grassy or swampy smell when you lift the lid.

White, cream or tan roots that look fuzzy are usually just healthy fine root hairs, not algae. Algae is green, and it grows where light falls.

Why algae grows

Algae are simple photosynthetic organisms. A Kratky nutrient solution is water full of nitrogen, phosphorus and the other elements plants need, so it is an ideal home for them as soon as light reaches it. In a kit, light usually gets in through:

• a clear or translucent reservoir, such as a glass jar or a thin plastic tub;
• gaps around the lid or net pot holes;
• empty net pot holes left uncovered;
• grow media at the top of the net pot that stays wet and sits in the light.

Why algae matters

How algae affects a Kratky kit

Effect	Why it matters in Kratky
Uses dissolved oxygen at night	Kratky roots have no air pump, so oxygen in the solution is already limited.
Uses nutrients	One fill has to last the whole crop; algae takes a share of it.
Coats roots	A slimy layer makes it harder for roots to take up water and air.
Decays	Dead algae feeds bacteria and fungi, which raises the risk of Kratky root rot.

A thin film on the wall of a jar rarely kills a crop on its own. Heavy algae in warm water is a different matter, because it combines several of the conditions that lead to root problems.

How to fix algae during a crop

1. Block the light first. Wrap the container in a sleeve, foil, paper or an opaque bag, and cover any open holes. Algae stops spreading once it is dark.
2. Leave light film alone. If the plant is healthy and the roots are white, covering the container is usually enough. Disturbing the roots mid-crop can do more harm than a little algae.
3. Move the plant for heavy algae. If the water is green and the roots are coated, lift the net pot, rinse the roots gently in clean water of a similar temperature, and set the plant in a clean, light-proof container with freshly mixed solution. Fill to just below the existing air roots rather than to the original fill line, so you do not drown them.
4. Clean the top of the media. Scrape off green growth on rockwool or clay pebbles, or add a thin layer of dry clay pebbles on top.

Avoid adding bleach or hydrogen peroxide to a planted kit unless you know the product and dose, because both can damage roots as well as algae. Cleaning products belong in the empty container between crops, as described in how to reuse a Kratky kit.

How to light-proof a Kratky kit

• Choose an opaque reservoir where you can: a dark tote, a bucket or a painted jar. The Kratky containers guide covers options.
• Sleeve clear jars. Fabric, paper, foil or a dark sock all work. Leave a slit if you want to check the water level.
• Cover unused holes with a spare net pot full of media, a cap or tape.
• Keep the lid sealed around each net pot.
• Keep grow lights on the leaves, not shining across the top of the reservoir.

Algae and boxed kits

Some boxed kits ship with clear or frosted containers because they look good on a counter. If yours does, it is worth adding a cover before the first fill. A kit with an opaque reservoir saves this step, which is one of the things to look for in the guide on how to choose a Kratky kit.

Frequently asked questions