NDSU Extension - Griggs County


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May 18

The Extension Connection

By Megan Vig

                Farmers are in the fields planting the 2018 crop and gardens are being prepped for seeds and transplants.  It is wonderful to feel the warm temps and know summer is on its way.  This week I share information from University of Nebraska Extension on hardening off transplants.

Outdoor conditions are very different than greenhouse growing conditions.  Transplants need time to adjust to these changes so you don’t lose plants or stunt them due to large amounts of damage.  Factors causing problems to new transplants include cold temperatures, wind, lower amounts of water and strong sunlight.  Fluctuating temperatures during spring is a given, so it is important garden transplants are prepared for the colder temperatures they face once planted outside.  Transplants grown indoors or in a greenhouse are very susceptible to damage if suddenly exposed to cold outdoor conditions.

                Wind is also responsible for much damage to early season transplants.  Strong spring wind can rip or tear leaves, or snap stems.  But even relatively low wind picks up small soil particles and throws them against the leaves and stems of young plants.  This type of damage is similar to sand blasting.  Leaf and stem cells are scratched and damaged, leading to greater moisture loss and tissue death.

                During the early stages of growth, transplants are given ideal conditions for growth including water wherever they need it.  Once outside, transplants experience conditions causing them to lose water faster, including heat, sunlight, and wind.  Finally, outdoor sunlight is often more intense than light provided by grow lights or in a greenhouse.  It can easily cause sunburn to the leaves of plants that are not hardened off.

                The goal of hardening off is to cause plant cell walls to thicken, changing soft, succulent growth to firmer, harder tissue.  Hardening off causes the following changes in plant growth and development.  Plant growth slows.  Natural waxes on leaf surfaces thicken as plants are exposed to more sun, reducing the rate of water loss.  Cell walls develop more lignin to strengthen them.  The amount of freeze-prone water in plant cells is reduced.  The amount of carbohydrates (stored food reserves) in plant tissue increases.  More rapid root development is stimulated.

                Harden your plants off gradually, over a 7 to 10-day period.  Ideally, start the process on a mild or cloudy day.  Place plants outside in a shaded location for 2 to 3-hours, sheltering them from strong wind.  Move the plants back inside at the end of this time.  Each day, gradually increase your plant’s time outside and their sun exposure by a few additional hours.  At the same time, gradually reduce how often you water, but do not allow the transplants to wilt.  After several days of 10 to 12 hours outside, leave the plants outside for 24 hours a day for a couple of days.  At this point, your plants are ready for transplanting. 

                Know the relative hardiness of your plants.  Hardy cool season plants can take temperatures in the 40’s.  After they are well hardened off, light frosts won’t hurt them.  Warm-season plants prefer warm nights, at least 60°F.  They can’t stand below-freezing temperatures, even after hardening off.  After these minimum night temperatures are exceeded go ahead and place transplants in the garden.

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