The cost of purchasing a landscape plant varies with the size of the pot the plant is in. Until recently, customers looked for quart containers, 1-, 5- and 10-gallon containers and flats of smaller plants. However, these names weren't accurate descriptions of the pot's capacity, so the names have been changed.
ANSI
The American National Standards Institute -- or ANSI -- now regulates pot sizes to standardize what size pot you are getting. Even indicating the pot's width, such as a 10-inch pot, doesn't tell you what the volume -- and therefore the potential root size -- of the pot is. One grower could use a taller 10-inch pot than another grower, resulting in different volume "10-inch" containers. The current ANSI standard for nursery pots was released in 2004.
Large Pot Classes
Manufactured pots have to fall within the ranges that define their classes. This allows for variations in heights and widths from different manufacturers to suit different growers needs, but still standardizes the volume. Manufacturers indicate large container classes by the pound sign, #, followed by a number 1 through 100. Manufacturers used to call these containers various sized "gallon" pots. With the new container system, the larger the number, the larger the container is. In cubic inches of volume, a #1 container -- which was commonly known as a 1-gallon pot -- is 152 to 251 cubic inches, a #2 container is 320 to 474 cubic inches, a #3 container 628 to 742 cubic inches and a #5 container -- which was commonly known as a 5-gallon container -- is 785 to 1242 cubic inches.
Smaller Pot Sizes
Small plant containers, usually holding perennials or annuals, are indicated by "SP" followed by the length of the side of the pot for square pots -- or the diameter, for round pots -- measured in inches. Since the length of the pot's side determines which category it falls under, manufacturers are limited in the pot heights they can produce and stay within the allowed volume. There are only five categories for small pots: #SP1 is 6.5 to 8.0 cubic inches, #SP2 13.0 to 15.0 cubic inches, #SP3 20.0 to 30.0 cubic inches, #SP4 -- formerly known as a quart container -- is 51 to 63 cubic inches and #SP5 is 93 to 136 cubic inches. Cell packs, like SP pots, must indicate the length of the side of the individual cell plus how many cells are in the tray.
What Size Pot to Buy
Nurseries sell annual plants in the #SP 1 through 5 sizes. The larger the pot, the more room the roots have had to grow and the sooner the plant will be able to flower. You can find lower-priced perennials in #SP 4 or #SP 5 pots, but they may not bloom the first year. Commonly, #1 containers hold second year perennials or young shrubs, while more mature shrubs are sold in #2 to #5 containers. The larger container sizes, such as #95, are for trees.
