January 19, 2013

1 - Warmth and supplemental light will help your houseplant cuttings form roots more readily in a cold room during the winter.

Yard and Garden January 19, 2013


I usually trim my houseplants in the fall when I bring them indoors and use the trimmings to start new plants. I was busy back then and never got around to starting new plants. I cut some stems from my coleus and begonias a few weeks ago and put them in jars of water as usual, but they are not forming roots. I put them on the windowsill in an unused bedroom with a window that faces east. What am I doing wrong?


Santa Fe


Coleus and begonia plants grow best when the temperature is warm. The windowsill in an unused room can become quite cold in the winter, especially if you close the heat vents and doors. At lower temperatures the plants will form their roots much more slowly, or not at all. Raising the temperature in the room may help, but nights may be too cold on the windowsill. Moving the plants away from the window to a warmer location that still receives bright light may help. You can also purchase thermostatically controlled heat pads to put under the jars in which you are rooting the cuttings. Set the heat pad thermostat to maintain a temperature of 70 degrees or higher. If the jar is large enough, a thermostatically controlled aquarium heater and an aquarium thermometer to help you regulate the temperature of the water will also work. More warmth should have a very positive effect on root formation.

Light is another factor to consider. The cuttings need light to produce foods through photosynthesis to feed the root formation process. In the winter the length of the light period is shorter reducing photosynthesis and, more importantly, altering the growth processes in some plants. Coleus and some begonias may be induced to produce flowers rather than roots under short day conditions. Supplemental light provided by a fluorescent lamp placed within a few inches of the plants will help. Turn on the light each evening before sunset and allow it to remain on long enough for the plant to experience 14 to 16 hours of light each day. A dark period of 8 to 10 hours will be sufficient. Some plants can tolerate constant light, but some plants require a dark period of a few hours. A timer controlling the lamp will make this job easier.

The heat pad and light may also be useful if you want to start vegetable transplants indoors. Warm season vegetables such as tomatoes and chiles need warm conditions to germinate properly. With the heat pad and lamps you can more successfully form roots on your houseplants and start vegetables to transplant into your garden for and earlier crop.

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: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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