Starting tomato plants indoors provides a head start on the growing season. A fundamental question for the home gardener is determining the optimal number of seeds to place in each cell of a starter tray. This decision balances guaranteeing a successful sprout against preventing overcrowding, which can compromise the growth of young plants. Optimizing seed use and promoting plant health from the beginning are necessary steps for cultivating a robust tomato crop.
The Standard Seeding Strategy
The general recommendation for starting tomato seeds is to plant two or three seeds per cell. This practice serves as insurance against the natural variability of seed germination rates. Planting a small cluster significantly increases the probability that at least one viable seedling will emerge in the space. This is particularly useful for gardeners who want to ensure every cell in their tray produces a plant without waiting for a second round of planting.
This method also provides the gardener with an opportunity to select for the most vigorous plant. When two or three seedlings emerge, they can be compared for stem thickness, leaf development, and overall robustness. By observing which plant is strongest after germination, the gardener can choose the best genetic material to nurture into a mature plant. Ultimately, placing multiple seeds in the cell is a strategic measure to allow for quality selection later.
Cell Preparation and Seed Placement
Before placing the seeds, the growing medium requires proper preparation to create an ideal environment for germination. Using a sterile seed-starting mix is advisable, and it should be thoroughly pre-moistened until it holds its shape when squeezed but does not drip water. This ensures uniform moisture distribution, as both dry pockets and waterlogging can inhibit the sprouting process. Once the cells are filled with the damp medium, the surface should be gently firmed down without compacting the soil excessively.
Tomato seeds should be planted at a shallow depth, typically about one-quarter to one-half inch (6–13 mm) beneath the soil surface. This shallow placement maximizes the seed’s limited energy reserves for emergence. Planting the seeds too deep can cause them to exhaust their stored energy before they can push through the soil to reach the light. After positioning the seeds, lightly cover them with the starting mix and gently tamp the surface to ensure good seed-to-soil contact for water absorption.
Post-Germination Care and Thinning
If more than one seed successfully germinates within the cell, the resulting seedlings must be reduced to a single plant to prevent detrimental competition. Seedlings that grow too close compete intensely for resources such as light, water, and nutrients, leading to weak or spindly growth. Allowing only one plant to remain ensures that all available resources are directed toward cultivating a single, strong specimen.
Thinning is performed when the seedlings have developed their first set of “true leaves,” which are distinct from the initial, smooth cotyledon leaves. True leaves have the characteristic scalloped shape of a mature tomato leaf, indicating the plant has begun self-sufficiency. At this point, the gardener should identify the weakest seedlings, often characterized by a thinner stem or less vigorous leaf growth, to be removed.
The preferred method for thinning involves using small, sharp scissors or snips to cut the unwanted seedlings at the soil line. Cutting the stem prevents the disturbance of the root system belonging to the remaining, chosen seedling. Attempting to pull unwanted plants out of the soil can inadvertently damage the fragile roots of the favored seedling, setting back its development. This careful removal process is necessary for cultivating a healthy, robust transplant.
Variables That Change the Planting Count
While the two-to-three seed rule provides a reliable baseline, certain variables can justify adjusting the initial planting count. Gardeners may choose to plant only a single seed per cell if they are using seeds that are expensive, rare, or known to possess an exceptionally high germination rate. Seeds from a reputable supplier that have been recently purchased and stored correctly often have a near-perfect viability, making the insurance planting unnecessary. This approach conserves precious seed stock.
Conversely, a gardener should increase the planting count to four or more seeds per cell if the viability of the seed stock is questionable. Seeds that are several years old, have been stored in less-than-ideal warm or humid conditions, or are of an unknown origin often have significantly lower germination rates. One simple way to estimate viability is through a germination test, where a small sample is sprouted on a moist paper towel to predict the percentage of successful sprouts. If the resulting germination rate is below 70%, overplanting with extra seeds is a practical adjustment to ensure a full tray of seedlings.