A horticultural flat is a standard, shallow, rectangular plastic tray, often referred to by the industry size of 1020, measuring approximately 20 inches by 10 inches. This tray serves as a carrier and watering base for smaller inserts that hold the plants. The final count of plants is a direct result of how the grower configures the cell packs inside this standardized space. The number of flowers in a flat is determined by the size of the individual containers held within the tray.
Common Flat Configurations
The count of plants in a flat is highly standardized within the nursery trade, ranging from 10 plants to over 100 in high-density setups. The most common configurations for bedding plants use “six-pack” inserts. A flat often contains eight packs of six cells for a total of 48 plants, which allows for convenient separation and transplanting of small annuals like petunias or marigolds.
For plants requiring a more established root system, the flat may be configured with fewer, larger containers, such as 10 to 12 four-inch pots. This arrangement offers a lower total plant count but a greater volume of soil per plant. Growers maximize the number of small seedlings using high-density plug trays, which can contain 72, 100, or even 128 tiny cells. These trays are used for young plants that need quick transplanting after purchase.
Factors Influencing Plant Count
The primary variable dictating the number of plants is the cell size, which relates directly to the required root volume for the species. Smaller, fast-growing annual flowers are grown in high-density trays because they complete their life cycle quickly and are not intended to be held in the tray for long. This smaller root volume encourages a high root-to-shoot ratio, which is beneficial for field establishment. Larger cell volumes are reserved for plants like perennials, vegetables, or larger flowering varieties that require more time to mature before planting.
The cell size also impacts the vigor and early yield potential of the transplant. Transplants grown in larger cells often exhibit greater height, leaf area, and dry weight at the time of field-setting. This increased size leads to greater early fruit or flower yields compared to those grown in smaller cells. Furthermore, fewer plants per flat means increased spacing, which promotes better air circulation around the foliage. Proper airflow reduces the risk of fungal diseases, particularly in susceptible plants such as zinnias and cosmos.
Translating Flats to Garden Space
Once you know the plant count, calculate the number of flats needed by considering the recommended spacing. Spacing varies widely based on the species, ranging from six inches for small annuals to 18 inches or more for large perennials. The goal is to space plants so they grow to touch at maturity, creating a seamless display without competing for light and nutrients.
To determine the number of plants needed, convert your garden bed size into total square inches. A 4-foot by 8-foot bed is 4,608 square inches. If plants require 8-inch spacing, divide the total area by the square of the spacing (64 square inches). This indicates you need 72 plants (4,608 / 64). Divide this count by the number of plants in your flat (e.g., 48 plants) to determine the number of flats required (1.5 flats).