David C. Zlesak, University of Minnesota Extension Educator
Taking plants from the relatively low light and moderated temperatures of the home environment and plunging them suddenly outdoors in bright sunlight, wind, and temperature extremes can result in severe injury. Depending on the extent of the injury the plant may be capable of recovering relatively quickly, after multiple weeks, or in extreme cases not at all. As plants grew indoors, the tissues that they produced were adapted to those environmental conditions. With a gentle transition period, plant tissue can adapt to some degree when conditions change.
Plants use environmental cues to help them adapt their tissues to their current growing conditions. For instance, leaves produced in shade typically have more surface area and are thinner in order to better intercept light and best invest energy resources into the most efficient type of tissue. Leaves grown under brighter light are often thicker with additional layers of photosynthetic cells within them and have a more developed waxy cuticle layer in order to better conserve moisture. Higher light levels are often associated with greater heat and therefore a greater potential for water loss. Leaves grown under higher light conditions also are typically higher in pigments like carotenoids and anthocyanins that serve in part to defend leaves from damage from excessive light, especially UV light.
Photo 1: These sweet peas were scorched by the sun and are severely injured. Notice the silvery haze over the leaf in the foreground and the completely sun bleached younger leaves. David Zlesak
When tissue adapted to one environment is suddenly transitioned to a vastly different environment, it can suffer injury. For instance, when we take houseplants indoors in fall where they experience lower light, leaves produced under the higher outdoor light levels often cannot suddenly adapt to lower light and have a tendency to fall off. In the spring we are generally forcing plants to endure the opposite transition by taking them from lower light into higher light levels. If the transition to higher light is too fast, the tissue can suddenly become scorched. Scorched tissue has a silvery white cast and is often irreparably damaged. It can remain silvery-white or soon may transition to brown and die. The chlorophyll, which causes plants to be green, can be damaged so severely by sudden high light (along with the cells housing this life sustaining pigment) that they no longer can undergo photosynthesis. Such plants will hopefully produce new foliage adapted to the new light conditions and recover.
Photo 2: These plants are taken outdoors on the deck for an increased time each day and are brought in at night. David Zlesak
In addition to severe and sudden changes in light levels, sudden changes in air movement, water availability, and temperature also can significantly shock plants. Plants in small containers can quickly dry out outside and wilt on a warm spring day and a dry wind can speed water loss. Wind can also whip young plants and result in bruised and broken tissue. Temperature extremes can also stress and stunt plants, especially low temperatures for warm season species. Many tropical species especially can experience chilling injury at temperatures below 50F. The lipids or fats in their cell walls are typically more saturated than those of temperate species. Lipids with greater saturation transition from a liquid to solid phase at warmer temperatures (like margarine, a relatively saturated fat, is a solid at room temperature, while corn oil, a less saturated fat, is a liquid). Solidification of lipids within plant membranes compromises their ability to function and can result in severe and in some cases irreparable damage. Because of this, it is not recommended to place warm season crops like tomatoes and squash out too early as cold temperatures can dramatically stunt their growth. Often later plantings of such crops can outgrow earlier planted, cold stunted plants of the same species, thus negating the benefit of trying to get a jump start on the growing season.
Fortunately, a gradual transition from indoor to outdoor conditions can allow changes to occur in existing plant tissue to allow it to better adapt to the new environmental conditions and then start producing new tissue more fully adapted to the new environment. Often a gentle hardening off/transition period of one to two weeks is sufficient to allow existing tissue to adapt to the extent possible. Here are some tips to harden off your bedding plants and houseplants you want to transition outdoors:
- Slowly transition plants to higher light conditions by incrementally increasing exposure to higher light intensity and duration at the higher light levels. Consider placing plants outdoors in bright indirect light first and then after a couple days begin to allow them to experience full sun. Slowly increase the duration in full sun over the hardening off period. One technique that can work well is to first place plants on the North side of a building where full sun is not experienced and then over time pull the plants further away from the building where there will be continually increased exposure to sun considering the time of day and shadows of the building. This is a useful technique especially for people who are away from home during the day.
- Check plants often for watering knowing that especially bedding plants in small pots can dry out quickly.
- Provide plants with protection from strong winds by placing them in protected locations. Tall or otherwise weak plants can benefit from staking to prevent stem breakage.
- For especially warm season crops, slowly transition them to cooler temperatures. Studies have pointed to the benefit of placing such plants in cooler temperature (~50F) conditions for only a couple to few hours a day during the transition period. This provides the signal to plants to adapt their lipids while limiting tissue injury.
Photo 3: The commercial greenhouse environment is often high in light and has good air movement. David Zlesak
Plants we purchase from the garden center are often grown in greenhouses with relatively high light and good air movement and the transition from that environment to our garden is less severe than from our home. Even so, sometimes a hardening off period can be beneficial, especially if we can tell that the plant was struggling for some time in the retail environment from perhaps being stored temporarily under a bench or in another less than ideal environment. Helping plants gently transition to their summer home will pay off in quicker adaptation and resumed growth.