December 22, 2012

1 - Light, low temperatures, limited nitrogen fertilizer, and a cold frame can help you grow better garden transplants.

Yard and Garden December 22, 2012


When I start my seedling indoors in the winter, they always get tall, but never become stout. They seem weak and do not do well when I transplant them to the garden in the spring. Is there something I can give them to help them become stronger?


Tall, leggy seedlings are often the result of too little light. That means not enough hours of bright light, not necessarily the brightness of the light. Full sunlight is not necessary and can often be a problem.

Somewhat filtered light through sheer curtains is often bright enough for the seedlings. This prevents the plants from forming “sun leaves” that are well adapted to bright sunlight, but that function poorly when the sun is not shining directly on them. The filtered light lets the plants form “shade leaves” that are more effective at lower light levels. The sheer curtains also diffuse light into the room at higher levels when the sun is not shining directly into the room and onto the plants. This gives more hours of sufficient brightness to the shade leaves of the seedlings. Before you move plants into the garden, you will need to harden the plants to the higher levels of light in the garden by slowly exposing them to brighter light over a period of a few weeks. This will also harden the plants to the temperatures and drier air found outside in New Mexico.

Excess nitrogen fertilization can also encourage leggy plant growth. Some nitrogen is needed for plant growth, but too much nitrogen results in problems.

Cool temperatures, especially at night will help develop stocky plants instead of leggy plants. Seeds need temperatures between 60 and 75 degrees to germinate. This depends on the varieties of seeds being germinated. Once the seed have sprouted and a few leaves are developed, they can tolerate lower temperatures and will become stockier as a result.

A cold frame outside (or a greenhouse) that will allow gradually increasing light intensities, cool night temperatures, and exposure to outside conditions while protecting the seedlings from freezing night temperatures is a great garden helper. A cold frame will allow you to move plants from your indoor starting location earlier and will allow your plants to become the best possible plants for transplanting into the garden once protection is no longer needed.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

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