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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

The recent arctic blast may have damaged and even killed some perennials and shrubs, but the key will be the length of time plants were exposed to freezing temperatures. The unique microclimate around your plant or shrub can influence cold tolerance by insulating plants from damage or exposing plants to even colder climates. Temperatures can be moderated slightly by protection provided by buildings, fences and concrete or rock mulches, which can absorb and retain, then release heat energy. This means stand-alone or exposed susceptible plants are much more likely to receive cold injury.

The cold of winter is the arbiter of what can and cannot be grown in our climate zone. However zone maps change over time and we all try to push into the next zone. When we see temperatures like this past week, even the “safe” plants in our gardens and yards may have suffered damage.

First let me explain what happens during a freeze. When the water inside a plant freezes it causes ice crystals to form that pierce the cell walls of the plant. When the temperature warms up, the cells leak out their fluids as they die and turn to mush. Freeze damage can first show up as dark, water-soaked tissues which then turn black to brown and dry up.

Most of the time in the fall or spring season gardeners are dealing with a marginal freeze where the temperature drops briefly to just below freezing at the end of the night and then moves back up above freezing soon after the sun rises. This is enough to destroy a fall or spring garden or fruit blooms and the hope of a spring crop. We can do a lot to protect plants from such a freeze because the temperatures are usually not too low and the duration is brief.

On the other hand when a hard freeze hits with a strong wind and lasts for a day or more there is usually little we can do to protect our gardens. The wind displaces any heat that might have helped protect the plants and speeds cooling of plant tissues. The extended time below freezing makes our simplest protective measures inadequate to the task.

Sometimes all we need to do is keep a plant alive through the cold. The first parts of most plants to freeze are the areas between leaf veins where the leaf is thinnest.  Keep in mind that plants vary in their cold hardiness as they develop from seedlings to mature producing plants.

There are a number of techniques we generally use to help avoid freeze damage to our plants.

All plants under drought stress can be more susceptible to cold damage. By watering plants several days or more before cold weather threatens you can relieve stress if they are suffering from drought. Water is also a great “heat sink.” That is, it holds warmth and releases it slowly, more slowly than plant surfaces or air. That’s why covering plants during a marginal freeze helps to protect them. Watering your plants right before a freeze creates a source of warmth that will slowly lose its heat over the course of a long cold evening. This alone is not going to provide protection from a hard freeze but every little bit helps.

Remember that plants growing in containers are especially susceptible to cold weather. Not only are the tops exposed like any other plant, but being above ground the roots lack the insulation of the earth and will get much colder than roots of an in-ground plant. Roots are often less hardy than the top portions of the plant. Some species which are normally quite hardy can suffer root death when temperatures in the container drop to just 28 degrees.

However, this week was not a marginal freeze, so what can we do to evaluate and mitigate the damage? Extension Horticulturalists note; first, do not panic at the miserable appearance of cold-sensitive plants just after a hard freeze.

Several factors will influence the extent of cold injury suffered by ornamentals and even certain types of fruit. Such factors include variety (some may be more cold tolerant than others), and age (recent plantings that are not well-established are more susceptible to freeze injury). A very important factor is the general health of a plant.

However, homeowners can take steps now to help reduce the occurrence of additional injures to ornamental and fruit plants resulting from the latest cold snap. These activities include the following:

• Keep plants well-watered. Watering is an extremely important plant-saving practice for winter.  It is very important that plants — those in containers, as well as in the soil — be provided adequate moisture throughout the winter season. The wind in the winter, like the sun in the summer, will dry soils.  Be especially sure that soils are well-watered if another cold snap appears to be forthcoming to prevent plant roots from drying out.

• Even though woody plants may appear to be in poor condition, do not do any pruning until late winter or early spring. Heavy pruning now can stimulate new growth which could easily be burned back if another cold snap occurs. Also, it is easier to prune and shape ornamentals after the full extent of damage is known.

• While proper fertilization is a key to winter hardiness for many perennial landscape plants. do not start fertilizing cold-stressed plants until they have resumed active growth in the spring. The use of fertilizer now may stimulate new growth, which is very susceptible to cold injury. Also, fertilizer salts may cause further injury to stressed root systems.

• Do not be in a hurry to prune plants. They can be cleaned up a little if they look unsightly or the neighborhood association sends a letter, but don’t cut these plants all the way back unless you’re willing to give up a security layer for the plant. Leave some of the damaged material intact. This will protect the plant from future freezes and will serve as a starting point for growth next spring.

• Plants with thick, fleshy roots like cannas, firespike, four o’clocks and gingers can be cut all the way to the ground, and they will regrow in the spring. Even after severe freezes, most plants like bougainvillea and hibiscus come back from the roots, so don’t give up on them.

• Even cool season vegetables may not  have fared well during the cold snap, check them once the temperatures have warmed up, you may have to replant if they were not protected.

• Some plants, of course, won’t stand any freezing weather regardless of how many toughening techniques you employ. That’s one of the reasons for using only thoroughly hardy plants in the basic framework of your landscape (such as for shade trees, and screening and foundation plantings).

The full extent of injury to many plants may not become apparent until summer.

Be aware that to reduce plant losses, it is important to take immediate steps now. It will be of utmost importance that cold-stressed plants also be provided good care throughout the 2011 growing season to safely achieve a full recovery.

In about a month the extent of the cold damage can be detected. Use a knife to scrape along the stems until you find green tissue. This is normally the point where the plant can begin new growth. For some, the green stems may be found only at the ground. Given time, even these plants can recover rapidly because of the well-established root systems.

For our wheat producers, the winter hardiness of most varieties when combined with good management practices have reduced winterkilling of wheat. Early-maturing wheat is more likely to be injured by freeze than is late-maturing wheat. While susceptibility to freezing temperatures steadily increases as maturity of wheat advances during spring, the dormant wheat plant is generally safe to the temperatures scene in this recent arctic blast.


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