Short answer: keep light out of the solution. Algae need light to grow, so an opaque container, a covered lid and covered gaps around the net cup will stop it.
Why algae is a problem
- It competes with your plant for nutrients.
- Its daytime photosynthesis can push pH up.
- When it dies and decays, it uses up oxygen and can encourage root rot and bad smells.
- It coats roots and container walls, making problems harder to see.
Where light gets in
- Clear or translucent containers, including glass mason jars
- White buckets and totes, which let more light through than they appear to
- Gaps between the net cup and the lid
- Exposed growing medium at the top of the net cup
How to lightproof
- Use opaque containers where possible. Kratky’s own trials used darkened bottles (Kratky, 2009).
- Wrap clear jars with aluminum foil, black tape, paint or a fabric sleeve. Leave a narrow strip you can uncover to check the level, or lift the sleeve.
- Cover white containers with a dark outer layer, or paint them.
- Seal gaps around net cups and cover exposed growing medium.
If algae has already appeared
A thin film is not an emergency. Lightproof the container now to stop it spreading. If the solution smells bad or roots look brown and slimy, move the plant to a clean container with fresh solution, or start again after harvest.
Between crops
Empty, scrub and rinse containers before reuse so you do not carry algae and microbes into the next crop.
Related guide: fixing Kratky nutrient solution problems.
Sources
- Kratky, B.A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65–72.
