What size net pots for hydroponics: lettuce, greens and bigger crops

For lettuce and leafy greens, a net pot of about 2 inches (5 cm) is what university growing guides use, set in a lid hole cut to the same size. University of Florida and University of Hawaii extension guides both specify 2-inch pots for lettuce, and Hawaii also lists tapered pots 1.5 inches (3.8 cm) across. For tomatoes and cucumbers the university work described below uses different containers, with no single net pot size you can read off a chart.

Why the pot matters less than people think

A net pot is a slotted cup that holds the seedling and a little medium above the water. It does not set the size of the root system. In the Hawaii suspended-pot method, the pots are tapered and slotted so roots can emerge, and the roots hang into the solution below. A leafy plant’s roots soon leave the cup through the slits and spend the rest of the crop in the reservoir. What you are choosing is a snug fit in the lid and enough room for the seedling’s cube.

That also explains why the pot is rarely where a problem starts. If lettuce is stalling, root and reservoir checks come first.

Lettuce and leafy greens: what the guides use

The numbers cluster tightly. University of Hawaii’s bucket guide drills three holes in the lid with a 2-inch hole saw “so the net pots fit snugly.” UF/IFAS Extension’s lettuce guide lists “2-inch net cups” with rockwool or foam grow cubes, and its video places those cups in a lid drilled with a 2-inch hole saw. Hawaii’s tray-and-panel design also puts each seedling in a 2-inch net pot.

Hawaii’s commercial-size tank design is a little smaller or squarer: tapered 5 cm by 5 cm pots, or 3.8 cm diameter by 7.6 cm deep pots, with leafy and semi-head lettuce spaced 20 by 30 cm. Purdue’s lettuce trial used net pots measuring 2.3 by 2.3 by 2.1 inches in a styrofoam sheet, spaced 8 by 6 inches.

Across the Hawaii, Florida and Purdue guides the lettuce pots run from 1.5 to 2.3 inches, with the 2-inch cup in the middle. The spacing between holes is a separate decision; transplant spacing for lettuce covers it.

Match the lid hole to the pot

Cut the hole after you have the pots in hand, and size it to the pot, as Hawaii’s 2-inch hole saw does for 2-inch pots. Hawaii’s paper on three non-circulating methods for lettuce also recommends tapered pots with slits, so roots can leave from the sides as well as the base.

Water level at the start

At transplanting, the University of Florida video shows the water about halfway up the net cup, so the seedling stays in contact with the solution. Hawaii’s guidance for a pot in a bottle neck is an immersion of the lower 3 cm. The level then falls as the plant drinks. In a Kratky jar, University of Minnesota Extension advises that only about half of the root biomass should touch the water once roots outgrow the pot, which is what keeps the roots oxygenated. In a pumped DWC system the air stone handles oxygen and the water level is managed differently.

Tomatoes, cucumbers and other big plants

Here the evidence changes shape. The University of Hawaii work on tomatoes did not settle on a net pot size. ‘Big Beef’ tomatoes yielded 2.68 kg per plant in 0.35 litre aluminium beverage cans. Tomatoes in 70 ml net pots gave similar yields in one trial and lower yields in another. Plants in 10 cm square pots holding 700 ml of perlite, resting on a nursery tray, yielded significantly higher than the cans in two trials. Hawaii’s explanation: the tray raises the pot, which exposes more root to moist air and gives roots a surface to form on and anchor.

Cucumbers in the same Hawaii paper went in a single tapered forestry tube, 3.8 cm by 21 cm per the University of Hawaii, with extra holes drilled in the sides, in a 3.8 cm lid hole on a 100 to 130 litre container.

What this means for you:

• The Hawaii tomato and cucumber work started plants in small tapered containers and then relied on a large tank, a pot on a raised tray or a long tube. The 2-inch lettuce cup is not a template for those crops.
• Plan the whole container, support and reservoir together. The hydroponic reservoir size calculator is the place to start, and the indoor tomato guide covers the rest of that system.
• None of these trials gives a single net pot size for fruiting crops, so treat any per-crop inch chart you find as a seller’s convention rather than a tested figure.

Medium inside the pot

The pot holds medium, and the medium decides how the cup behaves. A rockwool or foam grow cube suits seedlings, which is what the University of Florida guide uses. UF/IFAS rates expanded clay low in water-holding capacity. Rockwool vs clay vs coco vs foam compares them, so the comparison is not repeated here. Hawaii’s guidance for peat-perlite mixes is to tap the medium to settle it but not pack it so tightly that you restrict air.

A short way to decide

1. Leafy greens or herbs in a bucket, tote or jar lid: buy 2-inch net pots and cut 2-inch holes.
2. Panel, tower or kit built around a particular pot: use that pot, because the holes are cut for it.
3. Tomato, cucumber, pepper: size the container, support and reservoir first. Do not size from the net pot.

Sources

• University of Hawaii CTAHR, Growing lettuce in a 5 gallon bucket (Kratky)
• University of Hawaii CTAHR, Small-scale lettuce production with hydroponics or aquaponics (SA-2)
• University of Hawaii CTAHR, A suspended pot, non-circulating hydroponic method (Kratky, 2004)
• University of Hawaii CTAHR, Non-circulating hydroponic methods for growing tomatoes (Kratky et al., 2005)
• UF/IFAS Extension, “Set it and forget it” hydroponic lettuce (HS1488)
• Purdue Extension, Heating requirements for winter hydroponic lettuce production (HO-308-W)
• University of Hawaii CTAHR, Three non-circulating hydroponic methods for growing lettuce
• University of Hawaii CTAHR, Hydroponic vegetable production (VC-1)
• University of Minnesota Extension, Small-scale hydroponics