Avoiding Spindly Transplants

Gardeners often find it difficult to grow their own vegetable or flower transplants and frequently end up with spindly, weak plants that do not do well when placed outside. The two most common causes of spindly plants are low light and high temperatures after plants have germinated.

Unfortunately, these are the conditions commonly found on one of the most popular places to start seed — a windowsill. A windowsill doesn’t provide enough light, and temperatures are hard to control. In order to understand what is needed to grow transplants, the process must be broken down into three distinct phases.

Germination: Germination requires warm temperatures, and usually the seed does not require light (lettuce is an exception). Therefore, place seeded containers in a warm place even if there isn’t much light. For example, some people choose the top of a refrigerator. A heating pad is even better because most seeds germinate best at a constant temperature of between 70 and 75 degrees. This can vary by species with some requiring higher temperatures and others lower. Therefore a heating mat with a thermostat is helpful. Also remember that this temperature should be that of the media, not the air. Media temperature can be 5 to 10 degrees cooler than air temperature due to evaporation, which causes cooling. Covering containers with plastic can reduce evaporation (and temperature drop), and consequently, watering frequency. Check containers often and move germinated seed to a location with adequate light.

Growth: Light and temperature must be controlled during this stage to produce strong, stocky seedlings. Temperature should be lower than that used for germination, with 60 to 70 degrees preferred. Light must also be adequate for good growth. The easiest way for most people to provide what is needed is to use florescent lights. A standard two-bulb florescent fixture works well. It must be adjustable so bulbs can be positioned 2 to 4 inches above the top of the plants. Incandescent bulbs do not work well because they produce too much heat to be placed as close to the plants as needed. Also, the common florescent bulb produces the wavelengths of light needed. A grow light will not produce more or better growth.

Hardening Off: Plants grown indoors need some time to acclimate to outside conditions of wind and full sun. It usually takes about a week to harden off a plant. Reducing watering and temperature is key to toughen up transplants. If possible, move transplants outside for a portion of each day. Start by placing them in a shady, protected location and gradually move them into a more exposed, sunny location as the week progresses.
--Ward Upham, Kansas State Research & Extension Horticulture Newsletter