Extension News

Ask the ISU Extension Garden Experts: Crabgrass, Potatoes and Lilacs

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 e-mail us at hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, http://www.yardandgarden.extension.iastate.edu.


When should I apply a preemergent herbicide to my lawn to control crabgrass?

The key to successful control of crabgrass in lawns is the correct timing of the preemergence herbicide application. Crabgrass seeds begin to germinate when soil temperatures reach 55 to 60 degrees F and continue to germinate over several weeks from spring into summer. If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled.

Normally, preemergence herbicides should be applied in early to mid-April in southern Iowa, mid-April to May 1 in central Iowa, and late April to early May in the northern portion of the state. The timing of the preemergence herbicide application will vary somewhat from year to year because of weather conditions.

However, events in nature generally occur in a natural sequence. Preemergence herbicides should be applied when the forsythia blossoms start dropping or when redbud trees reach full bloom. Crabgrass seed germination typically begins after these events.

When should I cut my potato tubers into sections prior to planting?

Cut large potato tubers into sections one or two days before planting. Each seed piece should contain one or two "eyes" or buds and weigh approximately 1.5 to 2.0 ounces. After cutting the tubers into sections, place the freshly cut seed pieces in a humid, 60 to 70 degrees F location. A short, one to two day “healing” period allows the cut surfaces to callus or heal over. Callused seed pieces are less likely to rot in cool, wet soils.

I have oystershell scale on my lilac. How can they be controlled?

The oystershell scale is a common insect pest in Iowa. It can be found on a wide range of trees and shrubs. Plant hosts include ash, lilac, willow, maple, apple, pear, plum, cotoneaster, linden, and viburnum.

The oystershell scale is a small (1/20 to 1/8 inch long), elongated, oyster-shaped insect. The insect is concealed beneath a waxy, shell-like covering that varies from light to dark brown. Osytershell scale overwinters as eggs underneath the scale covering of the mother. In Iowa, eggs typically hatch from mid-May to early June. The tiny white to pale yellow “crawlers” move about on the plant looking for suitable feeding sites. The “crawlers” begin to develop a protective, waxy cover almost immediately after they begin to feed.

Oystershell scale attaches itself to the branches and trunks of trees and shrubs and sucks sap from plant cells. Heavy scale infestations may cause yellowing of foliage, stunting, and dieback of twigs and branches. Weakened trees and shrubs are vulnerable to additional insect and disease problems.

Contact insecticides do not penetrate the protective coverings of most scale insects. The best way to control oystershell scale is to apply an insecticide when the crawlers are present. Effective insecticides include insecticidal soap, horticultural oil, Sevin, permethrin, and others. The insecticide must be applied before the tiny insects develop their protective, waxy coverings.


Contacts :

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

Del Marks, Extension Communications, (515) 294-9807, delmarks@iastate.edu