A hydroponic garden in 40 m² with a small balcony: where to set it up, what to grow, what rules apply

No yard, no patio: a living room, a bedroom, a south-facing window and a balcony that fits two people on folding chairs. That is the situation of many readers, and it is exactly where hydroponics makes sense. No soil, no bag of potting mix to haul up to the fourth floor, and you can harvest salad greens all year round and cherry tomatoes in summer. The secret is not the equipment: it is putting each thing in the right place.

1. Take stock of your apartment

Before buying anything, walk around your home with three questions in mind: where is the light, where is there an outlet and a floor that can handle a leak, and where will the noise bother no one?

2. The standard layout: three zones, no more

In 40 m² (430 sq ft), it is tempting to put plants everywhere. Three well-equipped zones beat ten forgotten pots: fewer reservoirs to watch, less humidity, fewer cables.

• Zone 1, the balcony: from mid-May to October, for plants that love sun and heat. One tower or two tubs, no more.
• Zone 2, the windowsill: three to six Kratky jars of herbs, no pump, no noise. See the Kratky method in a jar.
• Zone 3, the LED shelf: the heart of the apartment garden. It produces all year round, whatever the weather.

3. Indoors: light comes first

It is the number one mistake in an apartment: thinking a bright room is enough. Our eyes adapt, plants don’t. Cornell CEA aims for about 17 mol/m²/day of light for lettuce in production. Behind a window, light drops quickly as soon as you move away from the glass, and in winter Belgian days are short and gray (Virginia Cooperative Extension, Iowa State).

• On the windowsill: enough from April to September for herbs; a 10 to 30 W LED bar takes over from November to February.
• In the room: one LED strip light per shelf tier, on a timer, 20 to 30 cm (8 to 12 in) from the leaves. See grow lights.
• In the evening: set the lights to turn off when you watch TV or sleep. White light is easier to live with than pink-purple lighting.

4. Indoors: humidity, noise and leaks

Humidity

Plants release into the air a good share of the water they drink, and an open reservoir evaporates. In a large home, that is negligible; in 40 m² (430 sq ft) already loaded by cooking, showers and laundry drying, it needs watching. Qualitel recommends 40 to 60% humidity in a home; above 70% for long periods, condensation and mold appear.

• A hygrometer next to the garden: the only way to know.
• Covered reservoirs: lid, drilled board, opaque film. That is also what keeps algae away.
• Air out every day, five to ten minutes, and don’t block the vents.
• Not against a cold wall: leave a few centimeters (an inch or two) between the shelf and an outside wall, where condensation forms first.

Noise

An air pump or a small water pump means a faint but constant hum. During the day you forget it; ten feet from the bed, at night, much less so. Set them on foam or a folded cloth, never directly on a shelf that resonates, and keep the bedroom for a silent Kratky jar. See DWC and air pumps.

Leaks

Hardwood floors are unforgiving, and in an apartment building the water ends up at the downstairs neighbor’s. Each system sits in a drip tray big enough to hold the entire reservoir, and outlets are higher than the water. See tubs, reservoirs and containers.

5. On the small balcony

The balcony brings something no apartment light can match: summer sun. It also brings constraints you don’t have in the living room. The balcony and patio guide covers them in detail; here is what matters on a small apartment balcony.

Weight

One liter of water weighs one kilo. A 40 L (10.5 gal) reservoir is 40 kg (88 lb) in one spot, plus the structure, the plants and you. Residential balconies are designed according to Eurocode 1; in France, the usual design value is 350 kg/m², about 72 lb/sq ft (Batiactu). That is an engineering figure, not a license to load: an old balcony, a cracked slab or exposed rebar calls for the opinion of the building manager or a professional. In any case, put the weight close to the wall, where the slab is supported, and choose several small reservoirs over one big one.

Wind

The higher you go, the windier it gets. A tower tips over from the top: tie it to a rigid point, on the inside of the railing (Oklahoma State University Extension). Nothing should stick out or hang on the outside of the railing: a planter falling from the third floor is no small matter.

Water and neighbors

Drain with a bucket, never over the railing. A drip tray under each reservoir, and an eye on the balcony drain: nutrient solution stains concrete and the balconies below. A leak into the neighbor’s home is your responsibility, and so falls under your home insurance.

Building rules and the lease

The co-ownership rules (in Belgium, the acte de base, or basic deed, and the règlement d’ordre intérieur, or house rules) often regulate the look of the facade, what can be seen from the street and what is attached to walls or railings. The lease can add its own rules. Read them before you drill: a system that stands on its own and is tied without drilling avoids most arguments.

Heat and electricity

• The reservoir in the shade: Cornell CEA recommends a solution that does not exceed 25 °C (77 °F). A south-facing balcony in July quickly heats up a black tub: umbrella, shade cloth, light color or a larger volume.
• A weatherproof outdoor outlet, protected by a 30 mA residual current device (the European equivalent of a GFCI), with the cable looping below the outlet. No extension cord running under the French door and getting crushed.
• Winter: empty, rinse and bring in pumps and reservoirs before the frosts. A full reservoir that freezes can crack.

6. Which system, where?

To compare systems in detail, see which system to choose. Microgreens deserve a mention: a tray on a shelf, a harvest in one to three weeks, it is the most profitable crop per square inch.

7. The year in Belgium

• November to February: the LED shelf does everything. Salad greens, mâche (corn salad), herbs, microgreens.
• March and April: sow the balcony’s cherry tomatoes, peppers and basil indoors. See sowing and transplanting.
• Mid-May: after the last frosts, move out to the balcony. Reassemble the system, test the pump, check every fitting.
• July and August: salad greens bolt quickly in the heat; keep them in the shade on the balcony or bring them back under LEDs. See tomatoes in hydroponics for the balcony plants.
• October: last harvest, bring the herbs in to the windowsill, empty, clean and put away the balcony system.

8. Budget and electricity

The formula: power (kW) × hours per day × 30 days × price per kWh. Three 30 W strip lights running 14 hours a day comes to 0.09 × 14 × 30 = about 38 kWh per month. At €0.45 per kWh all-in (Wallonia, September 2026, according to Selectra; €0.40 in Flanders, €0.46 in Brussels), count on about €17 per month in winter. An air pump or a small water pump adds a few watts. On the balcony, sunlight is free: one more reason to grow there in summer.

9. The most common mistakes

• Relying on room light: leggy, pale plants leaning toward the window. You needed a grow light.
• Buying everything the first month: start with a jar and a shelf, add the balcony in spring.
• The air pump in the bedroom: you unplug it at night, and the roots run short of oxygen.
• Open reservoirs: algae, gnats and humidity throughout the apartment.
• A freestanding tower on the balcony: the first gust of wind knocks it over, and the reservoir empties onto the neighbor’s.
• Forgetting the balcony system in November: a frozen reservoir cracks, a frozen pump has to be replaced.

Do you live in an apartment and already grow? Show off your setup on the forum: solutions found in 40 m² (430 sq ft) interest everyone.