A horticultural flat, often called a seed tray or 1020 tray, is a standard shallow container used by nurseries and gardeners to hold multiple smaller planting units. The number of plants in a single flat varies dramatically because the flat is merely an outer shell for various cell inserts. The total plant count is determined by the size and quantity of the individual growing compartments, which are chosen based on the plant’s needs and how long it will remain in the tray.
Common Plant Counts in Standard Flats
Flat configurations for home gardeners purchasing starter plants fit within the standard 10-inch by 20-inch outer dimensions of the tray. The lowest plant count is often the 6-pack, where six individual cell packs are arranged inside the flat, totaling 36 plants. Gardeners also commonly find the 18-count configuration, sometimes referred to as an 1801, which divides the flat into 18 slightly larger pots.
For annual flowers and larger vegetable transplants, counts range from 32 to 50 plants per flat, balancing quantity and cell size. High-volume growers and wholesale nurseries use denser configurations to maximize space efficiency. These include 72-cell, 102-cell, or 128-cell trays, where the small plants are called “plugs.” In specialized commercial settings, trays can be found with up to 288 miniature cells, primarily used for initial seed germination.
The Relationship Between Cell Size and Quantity
The principle governing the number of plants in a flat is the size of the individual cell, as all counts must fit into the same 200 square inches of space. A flat filled with 36 plants means each plant has a larger volume of soil and root space compared to one in a 72-cell configuration. The individual cells within a high-count flat are often shallow and narrow, defining them as “plugs.”
Plugs offer a small environment designed for quick germination and initial root establishment. Conversely, the larger cells of a 6-pack or 18-count tray are closer to small pots, accommodating a more extensive root system. This greater soil volume allows the plant to remain in the flat longer before it becomes root-bound. Root-bound occurs when the roots begin to circle the container walls. The choice between a plug and a larger cell is a direct trade-off between maximizing the number of plants and maximizing the growth time before transplanting.
Matching Plant Type to Flat Size
The cell count in a flat is selected to match the plant’s growth rate and root structure. Plants that grow slowly or develop deep, extensive root systems require larger, lower-count cells. Tomatoes, peppers, and broccoli benefit from deeper cells, often three inches or more, to support their taproots and prolonged growth indoors. A 32- or 48-cell tray is preferred for these vegetables to prevent premature transplant shock.
Plants with shallow, fast-growing roots, such as lettuce, basil, and small bedding annuals, thrive in high-count flats. They are successfully grown in 72-cell or 102-cell trays because they are intended to be moved to the garden soon after sprouting. Choosing the correct cell size ensures the plant has adequate nutrition and root development without wasting greenhouse space or materials. The cell size dictates the plant’s maturity level at the time of purchase.
Calculating Plants Needed for Garden Space
To translate the number of plants in a flat into garden coverage, a gardener must determine the recommended spacing for the mature plant. This spacing is usually provided in inches on seed packets or plant tags, dictating how many plants fit into one square foot. For example, a plant requiring 12 inches of space needs one square foot per plant. This means a 36-count flat will cover 36 square feet of garden area.
If a plant requires closer spacing, such as 6 inches, four plants fit into every square foot of garden space. In this scenario, the 36-count flat covers nine square feet of ground (36 plants divided by 4 plants per square foot). Gardeners should use the formula of dividing the total number of plants in the flat by the number of plants required per square foot to calculate the total area the flat will cover. This calculation is a step in planning a garden layout and purchasing the appropriate number of flats.