Too often, when reading seed packets, the phrase “thin seedlings,” appears with no further explanation. Most gardeners sow at least 3 seeds per hole or cell in a starting tray to ensure seed germination. Once the plants have begun to grow, they must be thinned, removing plants that are too close together to avoid harming the rest of the seedlings.
Benefits of Thinning
Limit your plants to one seedling per cell to limit the risk of disease. When plants grow crowded and their roots intermingle, disease spreads quickly and can cost you your whole crop. Removing extra seedlings gives them a chance to establish themselves before exposing them to pests and fungus that will impair their growth. Additional benefits include reduced fertilizer costs. Properly spaced plants do not strip the soil of its nutrients and experience less stress, increasing the yield.
Thinning Flower Seedlings
Check your seed sprouting trays for cells where more than one seed has germinated. Carefully remove the soil from around the new seedlings' roots and separate the seedlings. Prepare a second or third hole to receive the new plants in cells that did not have any seeds germinate. Quickly move one of the seedlings to the hole ready to receive it in the new cell. Repeat as necessary until only one plant is growing in each cell. This method works best for annual flowers and plants that bear fruits like tomatoes, peppers and eggplant. Thin the seedlings as soon as possible after germination. Water the seedlings once moved.
Thinning Root Vegetable Seedlings
Wait until seedlings have developed a second set of leaves before trying to transplant them to thin them. Root vegetables such as onions, beets, carrots and radishes do not develop roots that will get tangled in nearby sprouts very quickly. Allow them to grow a few extra days to strengthen their main taproot before gently removing them from their soil and moving them to a new hole. Water the transplant to settle soil and cushion the plant from shock.
Thinning Beans and Legumes
Cull seedlings from fast growing plants such as beans and peas. The fast-developing root systems and rapid top growth of these plants makes separating them from one another a risky proposition. Attempting to transplant a seedling that has grown entangled with another can kill both plants. Allowed to grow, even two legumes will deplete the soil in their immediate area and starve. Take a pair of clean garden shears and snip the weakest seedling in a group at or near the soil level. Investigate a few days later to check that it has not grown back.