When calculating the number of plants needed to cover a space, the requirements may be reported as how many flats to buy. A flat is a tray, usually plastic, made to facilitate watering and transporting groups of pots or plants. Some are simply large rectangular containers, while others may have small starter "cells."
Sizing Up the Flats
Most standard nursery flats, called "standard 1020s," measure about 21 inches long by 9 1/2 inches wide by 2 1/4 inches deep on the outside. The number of flowers or other plants in a flat can vary according to the size of the plants or the pots in which they're planted. The National Gardening Association says most nursery flats hold 50 plants.
Making Good Use of Flats
Carrying flowers home from the garden center is simpler in a flat because you can move several at once instead of carrying each pot individually. However, that's not the only reason to save the plastic trays. When you're not going to be able to transplant your new garden plants immediately or if you'll be unavailable to water your houseplants for a while, set the pots in the flat and add about an inch of water. The water will wick into the soil in pots with small drainage holes, and the evaporating moisture will keep plants happy and humid, as well.