Short answer: a rack with shelves about 48 × 18 inches holds two standard 10 × 20 trays per level, and 24 × 24 inches holds two in a smaller footprint. A common 36-inch-wide rack holds only one tray per level unless its shelves are at least 22 inches deep.
How many trays fit?
A standard 1020 tray is about 11 × 21.5 inches. The shelf's depth decides which way the trays can face.
| Shelf size (W × D) | Trays per shelf | 72-cell plants per shelf | On 4 tiers |
|---|---|---|---|
| 24 × 14 in | 1 | 72 | 288 |
| 36 × 14 in | 1 | 72 | 288 |
| 48 × 14 in | 2 | 144 | 576 |
| 24 × 24 in | 2 | 144 | 576 |
| 36 × 24 in | 3 | 216 | 864 |
Notice that a 36 × 14 shelf holds no more than a 24 × 14 one, since a second tray needs at least 43 inches of width. On a shelf 18 inches deep, trays still have to sit lengthwise, so the same rule applies.
Most apartment growers need far fewer than 288 seedlings. One or two trays can be plenty for a balcony or a few raised beds. Buy for the number of plants you can actually plant out.
Height between shelves
Leave about 16 to 18 inches between shelves. That allows for the tray, a humidity dome, the light and space for plants to grow. Racks with adjustable shelf heights make this easy.
What it costs to run
| Lit shelves | Total light draw | Monthly use | Monthly cost |
|---|---|---|---|
| 1 | 40 W | 19.2 kWh | About $3.50 |
| 2 | 80 W | 38.4 kWh | About $7.00 |
| 4 | 160 W | 76.8 kWh | About $14.00 |
Use your own rate in the grow light cost calculator. To light each shelf correctly, read our seedling shelf guide.
Apartment problems to plan for
- Leaks. Use solid trays with no holes under every cell insert. Put a waterproof mat under the rack. Renters can lose a deposit over water-stained floors.
- Humidity. Trays add moisture to a small room. Ventilate, remove domes after sprouting, and run a small fan.
- Electrical load. Use a surge-protected power strip suited to the total load, and keep it above the floor and away from drips.
- Light spill. Bright lights can disturb sleep. Use a closet or a corner with a timer set to your schedule.
- Pets. Anchor the rack if a cat might climb it. Some seedlings and soil additives are not safe to chew.
What to look for
Metal wire shelving
Strong and adjustable, and it gives you easy places to hang lights. Check the clear inner dimensions. See 48 × 18 wire shelves on Amazon.
Ready-made rack with lights
Saves time, but check actual watts and whether the shelves take standard trays. See racks with lights on Amazon.
Waterproof floor mat
Cheap protection for a rented floor. See waterproof mats on Amazon.
