Published on Wednesday, 30 November -0001 00:00
Judging from the number of questions I have been receiving, people are getting very anxious to transplant annual flowers and vegetable plants outdoors. It is still too early to transplant many of these tender plants outdoors even if there is no frost predicted. Chilling injury can stunt plants so that they never do perform up to their potential all season. In addition, cool, wet soils encourage root rot which not only stunts, but often kills the plants. I just lost two flats of celosia plants that I was hardening off in an unheated garage to root rot.
David Zlesak, Regional Extension Horticulturist, wrote the following article in the May 1, 2007, issue of Yard and Garden Line News. In this article, he cites the reasons to avoid chilling tender plants.
Is frost predicted tonight or in the extended forecast? This is an ongoing question that keeps gardeners glued to the local weather forecast, especially during April and May. For most of us, the answer strongly influences our gardening activities and priorities. It can help us determine if we will hold off on planting frost-sensitive transplants, or whether we need to declare a gardening emergency and rush out to cover frost-sensitive transplants already in the ground. Fluctuating spring temperatures may even set us to protecting established favorites already in active growth in order to avoid damage to flower buds and tender new foliage. Unfortunately, freezing damage (ice crystal formation that can rupture plant cells) is not all we should be concerned with. Chilling injury is also possible and is something gardeners rarely consider. Chilling injury is common especially on warm season annuals and plants from tropical and subtropical regions. Chilling injury can occur at temperatures from near freezing (32 degrees F.) up to about 45 to 50 degrees F.
Typical symptoms include overall slowed or stunted growth; watersoaked, wilted or dead regions on leaves, flowers, and fruit; and altered texture or flavor of susceptible fruits. Symptoms of chilling injury, especially stunted or slowed growth, can go undetected or be confused with other potential causes. Symptoms of chilling injury vary based on susceptibility of plant species and plant tissue, the degree of acclimation of plant tissue, the exact temperature(s) encountered, and the duration of exposure to chilling temperatures. Chilling injury is common in Minnesota, especially in bedding plants during stretches of cool spring temperatures and especially cool nights. It is also common in Minnesota with tropical or sub-tropical fruits and potted plants during winter months and primarily occurs when the plants are in transit. Plants may or may not recover from chilling injury depending on how much damage has occurred.
Unfortunately, many gardeners trying to get a head start on the growing season by planting their warm-season bedding plants early may actually lose the advantage of an extended growing season as plants use precious time trying to recover from chilling injury. Commonly-grown plants that are particularly sensitive to chilling injury include tomatoes, peppers, squash, dahlias, and impatiens. In fact, sometimes early planting may result in such delayed growth from chilling injury that smaller, later planted transplants may surpass earlier planted transplants suffering from chilling injury. I have especially noticed this with squash. The squash I direct seeded in the garden in late May have typically outgrown my earlier transplanted squash seedlings even though I try to protect my transplants from cold nights and chilling injury.