One gallon pots
The most common size at the garden center is a 1 gallon pot, referred to as a #1. In general round pots prevail over square Plant material consisting of flowers, trees, and shrubs from liners, cuttings, and flats typically go into #1's. Pot terminology is unique to the nursery trade. A 1 gallon pot only holds 3 quarts of soil in terms of liquid measure, but is still referred to as a #1. As for dimensions a #1 pot measures 6" wide by 7" deep and has a capacity of 183 cubic inches (6). Once the plant has been put into the pot, and some space has been left at the top for water, ten #1 pots can be filled with a cubic foot of soil. With 27 cubic feet of soil in 1 cubic yard, a nursery can pot up 270 #1 pots with a cubic yard of either soil or container mix. If #1's are placed pot-to-pot, in rows 12 pots wide, with 2.5 foot alleys between, approximately 2,700 #1's can be set out on 1,000 sq.ft. of ground.
Herbaceous perennial producers often use 3-6" square pots for smaller plants. Square pots are more space efficient that round. If perennials are to be sold in #1's they will need to transplanted from liner beds 6 months prior to sale in the spring.
Few tree seedlings are put into #1s as there is very little demand for such a small tree. The only exception would be the multitude of grafted Japanese maples. In the Pacific Northwest these small trees are a common sale item at spring garden fairs.
#2s, #3s, and #4s
These larger pots are especially useful for herbaceous perennials, and hybrid roses. Retail growers enjoy the sales of bedding plants grown in 2-3 gallon fiber pots, which are sold as color pots in the trade. Buy purchasing wholesale annuals in plug sizes, and fiber pots and potting soil from a nursery supply house, a part-time income can be generated during the months of March-May. By using an array of annuals grouped together in wide, relatively shallow container a grower create an excellent retail product for backyard decks and apartment complexes, with very little investment. When a fiber pot is selected with a wire hanger, growers can capitalize on the growing interest in hanging baskets filled with bountiful spring and summer annuals. Other common uses for 2-3 gallon fiber pots include the sale of budded hybrid T roses during the months of February-May.
#5s, through #15s and larger
A #5 container is the generally the minimum size for the sale of woody ornamentals at retail garden centers. This is also the minimum size for commercial landscape construction. Tree seedling liners are often put into #5s without first going into #1s. Larger liners look better from a retailer's viewpoint in a #5 container, as opposed to a smaller pot. A #5 container filled with mix and a plant weighs approximately 25-30 lbs., thus making it is still manageable by retail customers.
A #15 container can be an excellent choice for the smaller independent retailer looking to capture the higher end of the specialty garden trees such as Japanese maples, or weeping conifers. If growers have the time to allow these plants to attain a larger size they will be rewarded with a very high profit potential. Conifer growers who have dug their trees as B&B will often pot up any stock that did not sell in the spring into #15s for sale during the summer. A #15 pot, filled with a plant and soil will often weigh more than 90 lbs. so that a dolly and extra help will be needed for loading. Landscapers prefer #15s as they need at least 1" caliper trees for their installation projects. By having access to these larger container sizes they can plant good sized trees (6' or more) all year long, as opposed to field grown bare root material in the spring only. A complete guide to growing trees in #15s and larger trees is available from North Carolina State University (7).