BY ALLEN PHILO for Acres U.S.A. magazine
Avoiding transplant shock when transplanting starters from the greenhouse to the field is a key sustainable farming method.
The time of year has once again arrived when we will be taking plants out of the greenhouse and transplanting them into the field. This can be one of the most stressful experiences plants undergo as they are taken from the warm and sheltered environment of the greenhouse and placed into a field where they are at the mercy of the elements. Plants will almost always incur some amount of damage to their roots as well as their leaves during this process. All of these various stresses are grouped under the general name of “transplant shock.” If plants undergo too much transplant shock, it can leave them open to disease, pest pressure, and lower yield potential. But what can we do to help our plants through this period of increased stress?
Transplant shock is really the sum of all the stresses plants experience during the move from flat to field. In order to look at how we can help the plant through this time, we’ll divide these stresses into three different categories: environmental changes, physical damage, and nutritional deficiencies.
Most farmers help their plants acclimatize to these moisture and temperature changes by putting them through a period of “hardening off,” especially in the spring. This is done by taking the crop out of the greenhouse and placing it in a new location where the plant is exposed to air movement and greater temperature changes, but is still sheltered from weather extremes. This can be accomplished by locating the plants in an area where they are open to moderate breezes and lower daytime temperatures, but can be covered to shelter them from strong winds or nighttime frosts. This limited exposure signals them to strengthen their main growing stalks to cope with wind and change the chemistry of their leaves in order to withstand the lower temperatures.
To help transplants acclimatize to changes in soil temperature and biology and avoid transplant shock, there are several things we can do. The use of black plastic mulch in the field will warm the soil and is especially useful when it comes to cucurbits and solanaceous crops as it assists with weed control. Putting molasses into the transplant water can help too, as this will stimulate soil biology which in turn will raise the soil temperature.
The second and third categories of transplant stress, physical damage and nutrient deficiencies, are closely linked. Physical damage is unavoidable to a certain degree when transplanting. Care should be taken to avoid breaking any leaves or causing bruising as these injuries can become vectors for disease. The roots, however, not the upper part of the plant, often sustain the most damage during transplantation. Roots uptake nutrients mainly through their delicate root hairs and their growing tips, both of which are very susceptible to damage. This can lead to the plant experiencing a nutrient deficiency shortly after transplant due to its decreased uptake ability. This nutrient deficiency occurs at the same time the plant is trying to regenerate its root system and adjust to its new environment. This type of root damage can also happen easily with bare root transplants because in the process of removing the soil from the roots, more of the fragile root hairs can be damaged than when the transplants are in plug form.
Aiding Plants To Avoid Transplant Shock
Helping the plant through the transplant stress is essential and can be accomplished a number of different ways. One way is to stimulate the plant to grow with natural growth hormones. Another is to provide the plant with a supply of easily absorbable macro- and micronutrients. Kelp is an excellent source of natural growth hormones and micronutrients. During transplantation a liquid kelp extract works best as it can easily be added to water. It is also important to address macronutrients including phosphorus, calcium, potassium and nitrogen. All of these nutrients are involved in the formation of new tissue, and giving your plant an available supply of these nutrients will help it repair damage at a faster pace.
It is important to make sure that your plant is not already deficient in these nutrients before they go into the field. It is surprising how many plants have some phosphorus deficiency, noticeable by a purpling of the leaves, or a nitrogen deficiency, noticeable by yellowing or chlorotic growth, before going into the field. Plants deficient at transplant are at a further disadvantage since they are already struggling to make up for these nutrients as well as trying to repair damage. Make sure that you are using high quality potting mix for your seedlings to avoid this problem. Even with a good potting mix plants can become stressed, and it may be necessary to top dress the flats with a compost mix or fertilizer or you can inject liquid fertilizers into the for their needs. Special attention should be paid to plants that are past their ideal transplant date. Look for the noticeable signs of deficiency, and keep your plants well supplied with nutrition.
One of the best ways to decrease transplant shock is to supply extra nutrients and biostimulants at the time of transplant. There are several ways to accomplish this. One is to drench the plants while they are still in their flats. This can be done by mixing a large dose of nutrients into the final watering, or by mixing up a batch of “transplant soup” in a bin and submerging the flats in the solution until the soil is saturated. It is okay to have some of the “soup” get on the foliage of the plant as this will simply act as a foliar feeding. When dealing with bare-root transplants soaking the roots of the plants in a weak solution can be done instead. Another way to deliver this “soup” is to mix it into the transplant water. This works well, but depending on the transplanter, it can leave a lot of the solution in between the plants where it is not as effective. However it will help to stimulate soil biology, especially if molasses is used in the solution. Using the two systems of drenching flats and adding products to the transplant water works well, as it both provides the nutrition your plants need and stimulates soil life.
Transplanting is a very stressful time for the plants. They are put into conditions very different than what they are used to and are exposed to a wide range of stresses they have not encountered previously. The plants can also suffer damage during transplantation, especially to the root system, and this can lead to a period of nutrient deficiency as the plant tries to repair itself and as its ability to find nutrients has been decreased. All of these setbacks can weaken the plant and open it to disease and pest pressures, as well as decrease overall yield potential. By using conscientious cultural practices, stimulating root growth and soil life and giving the plant easily available forms of nutrients, we can help our plants pull through transplant shock faster. This in turn can lead to an increase in our plants’ ability to fend off disease and pests and result in improved yields.
Allen Philo has worked as the field operations manager on a large organic vegetable farm, and is currently the specialty crop consultant for Midwestern Bio-Ag.