The Kratky Method: Passive Hydroponics Guide

The Kratky method is passive hydroponics: you grow plants in a sealed container of nutrient water with no pump, no electricity, and no daily attention. A net cup holds the plant at the top, its roots dangle into the solution, and as the plant drinks the water down, an expanding air gap forms between the falling water line and the container lid. That air gap is the whole trick — roots in the gap pull oxygen from the air while roots still submerged drink water and nutrients. Done correctly, you set up a jar or tote once and harvest weeks later having added nothing in between. It is the single easiest entry point into hydroponics and ideal for windowsill lettuce and herbs.

How the Kratky method works

Plants need three things at the root zone: water, dissolved nutrients, and oxygen. Active hydroponic systems use pumps and air stones to keep oxygen in the water. Kratky achieves the same result without any equipment by exploiting the plant’s own water consumption. At setup, you fill the reservoir nearly to the bottom of the net cup so the growing medium wicks moisture up to the seedling. As the plant grows, it transpires and drinks, dropping the water level. The space left behind fills with air. The roots respond by differentiating: upper roots stay in the humid air gap and act like specialized “air roots” absorbing oxygen, while lower roots remain in solution feeding the plant. By harvest, the reservoir is mostly empty and the root mass spans the full container.

This elegance comes with limits. The system has a fixed volume of nutrients, so it only works for crops that mature before they exhaust the reservoir — which is why fast leafy greens are the perfect match and large fruiting plants are not.

What you need

• An opaque container with a lid — a half-gallon mason jar for a single herb, or a lidded plastic tote for a row of lettuce. Opaque matters; light in the reservoir grows algae.
• Net cups — 2 to 3 inch mesh pots that hold the plant and medium.
• Inert growing medium — hydroton (clay pebbles), rockwool, or a coco/perlite mix to anchor the seedling and wick water early on.
• Hydroponic nutrients — a proper hydroponic formula (commonly sold as a two-part A+B) plus a Cal-Mag supplement.
• A pH test kit and pH down/up — to keep the solution in range.
• A sunny window or grow light — leafy greens want strong light to stay compact.

That is the entire shopping list, and a single jar build can cost just a few dollars beyond the nutrients. If you also keep fish, the related but living-system approach in our aquaponics starter guide is the natural next step up in complexity.

Mixing the nutrient solution

This is where beginners go wrong. Ordinary garden or houseplant fertilizer does not contain the full spectrum of nutrients in soluble form that hydroponic plants need. Use a dedicated hydroponic nutrient at the label’s mixing rate (often expressed in mL per gallon or in target EC/PPM). For leafy greens, a target electrical conductivity around 0.8–1.2 EC is typical; herbs and fruiting plants want a bit more. Add a Cal-Mag supplement, especially if you use reverse-osmosis or soft water, since calcium and magnesium deficiencies are the most common Kratky problem.

After mixing, adjust pH. Hydroponic nutrients are best absorbed when the solution sits between 5.5 and 6.5. Test, then add a few drops of pH down (usually needed) to bring it into range. Lettuce and most greens are happy right around 5.8–6.0. Mix the full reservoir volume up front — the point of Kratky is that you fill once.

Step-by-step setup

1. Start your seedlings

Germinate seeds in rockwool cubes or a small plug of medium until they have a few true leaves and visible roots poking out the bottom. You can also transplant a young seedling, rinsing soil from the roots gently.

2. Set the net cup in the lid

Cut or use a pre-drilled hole in the container lid sized to your net cup so it sits snugly. Nestle the seedling and its plug into the net cup, packing hydroton around it to hold it upright.

3. Fill to the right level

Add nutrient solution until it just touches the bottom of the net cup — covering roughly the bottom third of the cup. The medium wicks moisture up to the small seedling, and there should already be a thin air gap. Do not submerge the whole cup; the seedling needs access to oxygen from day one.

4. Seal and place

Close the lid so no light reaches the reservoir, wrap the container in foil or tape if it is translucent, and set it under strong light. Then, essentially, leave it alone. As the roots grow down to chase the falling water, the air gap expands and the system self-regulates.

Best plants for Kratky

• Excellent: loose-leaf and butterhead lettuce, bok choy, spinach, kale, Swiss chard, arugula, basil, mint, cilantro
• Workable with a bigger reservoir: bush beans, small bush tomatoes, peppers (these need top-ups or a large tote because they drink more and grow longer)
• Poor fit: large indeterminate tomatoes, vining squash, root crops — too thirsty or too long-season for a fixed reservoir

Lettuce is the canonical Kratky crop: a head can go from transplant to harvest in roughly 4–5 weeks on a single jar of solution. For a layout to organize many jars or a larger grow, the small-space ideas in the urban gardening hub are worth a look.

Troubleshooting

No air gap forming / drowning roots: you filled too high and submerged the whole net cup, or the plant is too small to drink much yet. Pour off a little solution to expose part of the cup and let an air gap establish.

Algae in the reservoir: light is reaching the solution. Wrap the container in foil or use a fully opaque vessel and a snug lid.

Brown, slimy, or stinky roots: root rot from poor oxygenation, usually because the air gap never formed. Ensure the gap, and consider a few drops of food-grade hydrogen peroxide to add oxygen and suppress pathogens.

Yellowing or brown leaf tips: often a calcium/magnesium shortfall — add Cal-Mag — or a pH that has drifted out of the 5.5–6.5 absorption window.

Plant ran out of water early: the reservoir was too small for the crop. Use a larger container next time, or for thirsty plants, top up once with fresh solution rather than expecting a single fill to last.

Scaling up

Once a jar of lettuce works, the method scales beautifully. Swap the jar for a lidded storage tote, cut six to twelve net-cup holes in the lid, and grow a full salad bar from one reservoir. Stagger plantings a week or two apart so you harvest continuously rather than all at once — the same logic behind succession sowing in soil. Because there is no plumbing, you can tuck Kratky containers on windowsills, shelves, or under a single grow light, making it one of the most space-efficient and forgiving ways to grow food indoors.

When sizing a larger reservoir, plan on a rough guideline of about one gallon of nutrient solution per lettuce-sized plant for a full grow-out. A 27-gallon tote, then, comfortably runs a dozen heads with margin to spare. Space the holes at least 6–8 inches apart so the mature canopy does not crowd and shade neighboring plants. Mix the entire reservoir to your target pH and nutrient strength before planting, and resist opening or disturbing it afterward — every time you lift the lid you risk introducing light, pests, or contamination into the still solution.

Why Kratky is the best beginner hydroponics

Compared with the alternatives, Kratky removes nearly every point of failure that frustrates new hydroponic growers. There is no pump to clog or burn out, no air stone to clean, no electricity bill, and nothing that fails silently during a power outage and drowns or starves your plants. The only ongoing variables are light and the initial solution, both of which you set once. That simplicity makes it the ideal first system to learn the fundamentals of soil-free growing — pH, nutrient strength, and the relationship between roots, water, and oxygen — before investing in pumps, reservoirs, and controllers. Master a jar of lettuce, and you understand the core of every hydroponic method, including the living, fish-powered systems covered in our aquaponics guide.