Hydroponics troubleshooting: root rot, algae, leggy plants, pests and deficiencies

TUREFF• •0

Hydroponic plants grow fast, and problems move fast too. A root problem in soil can take weeks to show; in water it can turn in a couple of days. The good news is that most problems trace back to a short list of causes: water temperature and oxygen, light, nutrient strength and pH, and hygiene. Work through them in that order.

Root rot

Signs: healthy roots are usually white or cream and firm. Rotting roots turn brown, soft and slimy, may smell unpleasant, and the outer layer slips off when you pull gently. Plants wilt even though they are standing in water, because damaged roots cannot take it up. Causes: low oxygen at the roots, often from warm water, a failed or missing air pump, or roots sitting in stagnant water. Water moulds such as Pythium are a common cause and take hold more easily in roots already stressed by low oxygen. What to do:

Remove badly affected plants so they do not contaminate the rest.
Trim away rotten roots with clean scissors.
Replace the solution, clean and rinse the reservoir and equipment.
Improve aeration: check the pump and air stone, or lower the water level to leave more air around the roots.
Keep the reservoir cool: out of direct sun, away from heaters, in an opaque or insulated container.

Prevention matters more than cure here. Losses can happen quickly once rot takes hold.

Algae

Signs: green film on the medium, slime in the reservoir, green-tinged water or tubes. Causes: light reaching the nutrient solution. Algae compete for nutrients and oxygen, can clog pumps and tubes and can harbour other problems. What to do:

Use opaque reservoirs, lids and tubing, or wrap clear ones.
Cover exposed medium around plants, and fill unused net pot holes.
Clean thoroughly at each solution change.

Leggy plants

Signs: long, thin, pale stems with wide gaps between leaves, plants leaning or flopping. Causes: almost always too little light, sometimes made worse by warmth. Seedlings stretch quickly if light is too far away or too weak. What to do: move the light closer, add a grow light, or extend the light hours within sensible limits. A small fan gently moving the air helps stems firm up.

Bolting

Lettuce, coriander, rocket and spinach can switch to flowering, growing a tall central stem and turning bitter. Heat and long days encourage it. Grow bolt-resistant varieties, keep the area cooler, and harvest promptly.

Pests

Hydroponics avoids soil pests but not all pests. Common indoor ones include aphids, spider mites, thrips, whitefly and fungus gnats, which breed in damp media and algae.

Inspect new plants and seedlings before they join the system.
Check under leaves regularly.
Yellow sticky traps help monitor flying pests.
Wipe or rinse small infestations off by hand, and remove badly affected leaves.
If you use a treatment on edible crops, check the label says it is suitable for edibles and follow the harvest interval. Never add products to the nutrient solution unless they are designed for it.

Nutrient problems

Before blaming a specific nutrient, check pH and EC. Most apparent deficiencies in hydroponics are caused by a pH that has drifted out of range, which locks nutrients out, or a solution that is too weak, too strong or too old. Common patterns, as a general guide:

Older, lower leaves yellowing evenly: often nitrogen shortage.
Yellowing between the veins of new leaves: often iron, frequently linked to high pH.
Yellowing between veins of older leaves: often magnesium.
Brown, burnt leaf tips on lettuce: tip burn, usually calcium not reaching fast-growing leaves, often worse with a strong solution, heat or poor airflow.
Burnt edges across many leaves, crusty deposits: solution too strong.
Slow growth and pale colour overall: solution too weak or light too low.

Changing the whole solution fixes many of these at once.

A checklist when something goes wrong

Look at the roots.
Measure pH and EC.
Feel the water temperature.
Check the pump, air stone and timer.
Look at the light: distance, hours, whether it is working.
Check under the leaves for pests.

Post a description or photos of the problem, with your readings, in the comments and others can help you work out what is going on.