A closed system has no weeds, no slugs and no diseases overwintering in soil. What remains are two categories: fliers that come through the window or arrive on a bought seedling, and pathogens that travel through the water. Both are manageable long before they become visible — if you know what to look for.
The five pests you will actually meet
| Pest | Size and look | First sign | Biological control |
|---|---|---|---|
| Fungus gnats (Sciaridae) | 3–4 mm, black, jerky flight | Gnats above the substrate, larvae in the cube | Bti (Bacillus thuringiensis israelensis) in the water, yellow traps |
| Aphids | 1–3 mm, green or black, in colonies | Sticky honeydew on the leaf below the colony | Parasitic wasp Aphidius colemani, lacewing larvae |
| Spider mites | 0.5 mm, barely visible, webbing later | Fine pale stippling on the upper leaf surface | Predatory mite Phytoseiulus persimilis |
| Thrips | 1–2 mm, narrow, run away fast | Silvery feeding scars and black frass dots | Neoseiulus cucumeris, blue traps |
| Whitefly | 1–2 mm, white | A cloud rising when you shake the plant | Parasitic wasp Encarsia formosa, yellow traps |
Fungus gnats are the one pest hydroponics partly solves by itself: the larvae need permanently moist organic material. If the top layer of rockwool or coco is dry and the tank is closed, the cycle breaks. In a vertical tower the exposed substrate area is tiny anyway — cover the net pot openings and there is nowhere left to lay eggs. The development cycle takes about 3–4 weeks at 22 °C, so one month of consistency clears the population without a single product.
Spider mites are about humidity, not hygiene
Spider mites are the classic indoor winter mistake. At 30 °C and 40 % relative humidity one generation takes roughly 7 days; at 20 °C and 60 % it takes about 20. That is the difference between eight and three generations in two months. Push humidity above 60 %, keep temperature below 26 °C, and the population collapses on its own. Check leaf undersides with a 10× lens — you will see the mites a full week before the first webbing.
Biological defence: what actually works
- Yellow sticky traps — not a treatment but an instrument. Count the catch weekly; a jump from 3 to 15 gnats means something is starting.
- Blue traps — for thrips only, they barely respond to yellow.
- Steinernema feltiae nematodes — against fungus gnat larvae, roughly 0.5 million per m² or straight into the nutrient solution; keep the water below 25 °C or they die.
- Predatory mites — Phytoseiulus persimilis, 2–6 per plant at first sighting, repeat after 14 days.
- Potassium soap 1–2 % — the fastest first aid against aphids; spray the underside, repeat after 5–7 days because eggs survive.
- Neem (azadirachtin) — slow, works through the moulting cycle; spray in the evening and never let it reach the tank, since it leaves an oily film on the water surface.
Being honest about it: beneficials in a flat have no alternative food and disappear once the prey is gone. So you release them deliberately at first sighting, not preventively every month.
Diseases: root rot is problem number one
Pythium and Phytophthora are not the disease of a sloppy grower — they are water organisms waiting for one single condition: lack of oxygen in the solution. And oxygen leaves the water as it warms up.
| Water temperature | Maximum dissolved oxygen | Rot risk |
|---|---|---|
| 16 °C | ≈ 9.9 mg/l | very low |
| 20 °C | ≈ 9.1 mg/l | low |
| 24 °C | ≈ 8.4 mg/l | borderline |
| 28 °C | ≈ 7.8 mg/l | high |
| 32 °C | ≈ 7.3 mg/l | very high |
These are theoretical saturation values; roots and microorganisms consume oxygen, so the real figure is lower. Below roughly 5 mg/l the root stops respiring normally and Pythium has an open door. Healthy roots are white to cream, firm and odourless. Brown, slimy roots that mush between your fingers and smell like a pond are already infected.
In practice: never put the tank in direct sun, drop in a frozen 0.5 l bottle on summer mornings, and keep the lid closed and lightproof. Darkness also prevents algae, which consume oxygen at night and make the same problem worse. More on temperature and hardness in the article on water quality.
Powdery mildew and botrytis
Powdery mildew (white floury coating on the upper leaf surface) likes 20–25 °C, high night humidity and dry leaves — typical for basil and cucumber indoors. Your first move is air: a small fan keeping the leaves gently moving reduces infection more than any preparation. On the first spots, potassium bicarbonate at 3–5 g/l or skimmed milk diluted 1:9 every 5–7 days helps.
Botrytis works the opposite way: it needs above 85 % humidity and dead tissue as an entry point — old lower leaves, stem stubs left after harvest. That is why removing yellowing leaves and cutting cleanly without stubs pays off.
Prevention is 90 % of the job
- Keep new plants 10–14 days away from the tower — most pests arrive on a bought seedling.
- Never put a soil-grown plant into the system without washing the roots thoroughly.
- Five minutes once a week: leaf undersides with a lens, count the traps, smell the tank.
- Spacing: 32 slots do not mean 32 large plants. Tomato and cucumber want the neighbouring slot empty.
- Rinse tank, pots and column between cycles — the procedure is in the article on tower maintenance.
Symptom → cause → action
| Symptom | Likely cause | First action |
|---|---|---|
| Fine pale stippling, webbing later | Spider mites, dry warm air | Humidity above 60 %, rinse undersides, predatory mites |
| Sticky shine and curled shoot tips | Aphids | Potassium soap on undersides, repeat after 5–7 days |
| Silvery streaks and black dots | Thrips | Blue traps, Neoseiulus cucumeris |
| Black gnats around the net pots | Fungus gnats, surface too wet | Dry the surface, Bti in the solution, yellow traps |
| Brown slimy roots, pond smell | Pythium — water too warm, too little oxygen | Cool below 22 °C, aerate, replace the solution |
| White floury coating on upper leaves | Powdery mildew | Fan, potassium bicarbonate 3–5 g/l |
| Grey fuzz on stem or fruit | Botrytis, humidity above 85 % | Remove affected parts, ventilate, lower humidity |
| Yellow leaves with no pest in sight | Not a disease — solution chemistry | Measure pH (5.5–6.5) and EC |