How to Prevent Leggy Vegetable Seedlings

Leggy seedlings are asking for more light

leggy vegetable seedlings

The Spruce / Margot Cavin 

Most serious veggie gardeners get a jump on the growing season by starting seeds indoors. In cooler climates, starting seeds inside assures that slow-maturing plants like tomatoes, cucumbers, zucchini, and squash will produce midsummer fruit. This tactic works for flower gardeners too, as buying flower seeds is an economical alternative to purchasing young perennials and annuals.

However, indoor seed starting can be fraught with difficulties, the most common being legginess. Even seeds that are carefully sown in the proper soil, watered precisely, placed in a south-facing window, and watched diligently until they germinate and sprout can fall victim to weak and spindly stalks. This irregular growth pattern, known as "legginess," is unhealthy for your plant as it grows larger, making it unable to support its weight. Also, leggy seedlings can face great challenges once exposed to outdoor elements. So, before transplanting seedlings outside, make sure your indoor conditions create well-established plants that will thrive in a garden setting.

Causes of Leggy Seedlings

Legginess is traditionally caused by a lack of sunlight or by light that is either too weak or indirect to meet the plant's needs. The stems grow thin because the plant is desperately reaching its leaves towards the light. Seeds that are started late winter or early spring, when the sun is still low in the sky, are prone to this type of growth pattern. During this season, the sunlight is barely strong enough to allow plants to grow at all, much less thrive.

Preventing Leggy Seedlings

There are three surefire ways to prevent your seedlings from becoming leggy and they all involve light manipulation.

  1. Provide direct light: Just because a window seems bright from sunup to sundown does not mean it yields enough direct sunlight to grow plants. Seedlings need to be placed in a window that experiences six to eight hours of direct sunlight each day. Even plants that thrive outdoors in partial shade during the summer need direct sunlight to propagate seedlings.
  2. Provide artificial light: If your window of direct light falls nowhere near the needed six to eight hours, artificial light may be warranted. A variety of specialty grow lights are available for purchase, but you can certainly get by with an inexpensive fluorescent shop light. Use this light to provide at least 16 hours of light for your seedings per day. Enlist the help of a timer to make it easier to keep track of the exposure time without any additional effort.
  3. Adjust supplemental light: Lastly, your supplemental light source needs to be close enough to your plants so that they are not reaching out to grab it. Two to three inches from the top of the seedlings should suffice (make sure not to burn the leaves). A distance further then that will make the tiny plants stretch themselves, resulting in legginess.

Once the seedlings establish themselves and contain many sets of leaves, pinch off the top set to keep the plants compact and train them into a bush.

Preparing Seedlings for Outdoor Growing

A common mistake of beginner green thumbs is sowing tender indoor seedlings directly into the garden as soon as the recommended planting date arrives. This can lead to a disaster, as the sudden change in environment can kill unacclimated seedlings. To avoid this loss, begin "hardening off" your seedlings before their transplant date by moving them outdoors for a few hours on days with mild temps and light breezes. Then, bring the plants indoors at night to protect them from the cold. Gradually increase the length of time they spend outdoors during the day. By the time you are ready to plant your garden, the seedlings will be tough enough to survive any inconsistency in the weather.

Article Sources
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  1. Clarke, Ethne. Seed Starts & Smarts: An Organic Gardener's Guide to the Fundamentals of Growing Plants from Seed. Potter/Ten Speed/Harmony/Rodale, 2012