A Kratky garden is a simple way to grow a plant. There is no pump and no electricity.
Kratky Problems
Kratky algae grows for one simple reason: light is reaching the nutrient solution. Algae are simple plant-like organisms, and a container of water full of nutrients is an ideal place for them, as long as they get light. Take away the light and algae cannot grow.
That is why every Kratky guide asks for a dark container and a lid that blocks light. A clear jar on a sunny windowsill is the most common place to see algae.
A little green on the top layer of clay pellets, where light falls on damp surfaces, is common and less of a worry than algae in the solution itself.
Algae competes with the plant for the nutrients in the solution. It also uses oxygen, especially at night, and can coat the roots, which makes it harder for them to take in water and air. As algae dies off, it breaks down in the water, which can encourage root problems. A container full of algae is not a healthy place for roots.
The fix is to stop light reaching the water. Wrap a clear jar in aluminum foil, or slide it into a dark fabric or paper sleeve. Paint the outside of a container with an opaque paint, keeping the paint off the inside. Choose opaque buckets and totes, not translucent ones. Hold the container up to a bright light: if you can see light through it, algae can grow in it.
Leave a narrow strip you can peel back if you want to check the water level, and cover it again afterward.
Light also gets in from above. Make sure the net pot fits the lid snugly, and cover any gap around it. Fill the net pot with enough clay pellets that light cannot shine straight down into the water. On a larger lid with spare holes, cover the unused holes with tape or a cap.
If algae has already started, block the light first, then decide whether to clean up now or wait. With a plant that is close to harvest and growing well, simply blocking the light is often enough. Early in a grow, or if the roots are coated and the water smells, it is better to start fresh.
To start fresh, lift the plant out, rinse its roots gently in clean water, wash the container and lid in hot soapy water, rinse well, and set it up again with fresh solution as in Step 2: Add water and nutrients. Keep the plant's roots damp the whole time it is out of the container.
Wash containers, lids and net pots in hot soapy water and rinse well before reusing them. Rinse clay pellets until the water runs clear. Leftover algae on a container or pellets gives the next batch a head start.
Algae and root rot often appear together, because both are encouraged by warm water and stagnant conditions. If you see algae, also check the roots and the water temperature. Kratky Root Rot covers what healthy and unhealthy roots look like.
More on choosing a container in Step 1: Pick a container, and every other common issue on Kratky Problems.
Algae in the container does not usually reach the leaves you eat, but a container full of algae is a sign the setup needs fixing. Wash the leaves well before eating, as with any harvest, and discard any that look slimy or moldy. If the roots or solution smell bad, start over in a clean container rather than harvesting from a struggling plant.
Green on the top layer of clay pellets is common because light falls on the damp surface. It does little harm, but you can reduce it by adding a thin top layer of dry pellets or covering the net pot surface. Between plantings, rinse and dry the pellets before reuse.
Tip: Hold your container up to a bright window before you plant. If light shows through, wrap it.
Nutrients, pH and safety guide