Extension News

Ask the ISU Garden Experts: Freezing Temperature Damage to Potatoes, Rhubarb and Fruit Trees and Propagating Mums

Note to media editors: Got gardening questions? Call the Hortline at 515-294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or send an e-mail to hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, www.yardandgarden.extension.iastate.edu.


Freezing temperatures damaged my potatoes.  Will I need to replant? 
Potato shoots (stems) are sensitive to freezing temperatures. Symptoms of freeze damage may vary from blackening of the leaf margins (minor damage) to death of all above ground growth (severe damage). Fortunately, severely damaged potatoes will send up new growth (shoots) within 10 to 14 days. There is no need to replant the potatoes. 

Is it safe to eat rhubarb after the plants have been exposed to freezing temperatures?
After freezing temperatures, some gardeners express concerns about the edibility of rhubarb. Rhubarb is a tough plant. Temperatures in the upper 20s or low 30s usually cause little or no damage. A hard freeze (temperatures in the mid-20s or lower) is usually required to cause serious damage. Rhubarb damaged by freezing temperatures will have black, shriveled leaves and soft, limp leaf stalks. It’s safe to harvest rhubarb if the plants show no signs of damage two or three days after the freeze event. Damaged rhubarb stalks (blackened foliage and limp stalks) should be pulled and discarded. New stalks that emerge after the freeze are safe to harvest. 

Will a late freeze in spring affect my apple crop? 
A late freeze in spring may damage the blossoms or developing fruit on apples, cherries and other fruit trees. As a result, the fruit crop may be smaller than normal. The extent of damage will be determined by the plant species, stage of flower/fruit development and temperature. At petal drop on apples, a temperature of 28 degrees Fahrenheit will kill approximately 10 percent of the developing fruit while a temperature of 25 degrees will kill approximately 90 percent of the developing fruit. 

How do I propagate garden mums? 
Chrysanthemums can be propagated by division and rooting cuttings.

Divide mums in early spring just as new growth begins to appear. Dig up the entire plant and divide each plant clump into sections with a sharp knife. Each division should contain several shoots and a portion of the root system. Immediately replant the divisions. 

Collect cutting material in spring or early summer. When the new growth is several inches long, cut off the upper 3 to 4 inches of the shoots with a sharp knife. Pinch off the leaves on the bottom portion of the cuttings. Dip the bottom of the cuttings in a rooting hormone. Then stick the cuttings in a rooting medium (coarse sand or perlite). Keep the rooting medium moist. The cuttings should root in four to five weeks. When the cuttings have good root systems, remove them from the rooting medium and transplant the rooted cuttings into pots or plant directly into the garden.

Contacts :

Richard Jauron , Horticulture, 515-294-1871, rjauron@iastate.edu

Willy Klein, Extension Communications and External Relations, 515 -294-0662, wklein@iastate.edu