May 4, 1994

Edibility of Rhubarb

The below freezing temperatures across much of Iowa last week has caused some gardeners to question the edibility of their rhubarb. Their concern is that the poisonous oxalic acid in the leaf blades has moved down (leaked) into the stalks upon exposure to freezing temperatures. The movement of large amounts of oxalic acid into the stalks is high unlikely. Rhubarb is a cold tolerant plant and generally is not damaged by a light freeze. A hard freeze will damage the plant.


Astilbe, also known as false spirea or meadowsweet, is a shade tolerant perennial suitable for many Iowa gardens. Most astilbe cultivars grow 2 to 3 feet tall although some varieties can grow to a height of 4 feet. The mid to dark green foliage is ternately compound (divided into 3 parts). Some varieties have a reddish-copper overcast to the foliage. The flower stalks are erect or arching panicles up to 2 feet long. Bloom occurs from early June through August depending upon the cultivar. Flowers are available in white and various shades of pink, red, or lavender.

The Effect of the Recent Cold on Flowering Fruit Trees

The cold temperatures last week in Iowa may have damaged some of the blossoms (ovaries) on fruit trees. The extent of damage depended upon the stage of flower bud development and minimum temperature at the site. Flower buds become less resistant to cold injury as they progress from the silver bud stage to full bloom. Many of the fruit trees in central Iowa were in full bloom last week. At the full bloom stage, previous scientific studies have found that only 10% of apple blossoms are killed when the temperature drops to 28 degrees Fahrenheit.

Environmental Injury on Conifers

Not all browning and loss of conifer needles is caused by infectious diseases. Environmental and site stress can also cause discoloration and death of needles.

Common Conifer Diseases

Many conifer samples showing needle diseases have been arriving in the Plant Disease Clinic. Most of these diseases can be diagnosed by carefully examining needles. Control of these diseases requires the use of protectant fungicides when the new growth is emerging (May).

IPM Update -- Apples and Strawberries

The 1993 growing season started out like summer, but the last week has seen a return to early spring conditions, even some snow in parts of the state. Some early-season disease management observations and suggestions:


Pests to Watch for on Ornamental Trees and Shrubs

Eastern Tent Caterpillar.

Now is the time when we should first begin to notice the silvery-gray webs of the eastern tent caterpillar on apples, crab apples, wild plum, cherry and related trees. Larvae should be emerging from eggs that were laid on small twigs last summer by the female moths. These larvae gather at a major branch fork or crotch and begin to build the silk web or tent.