In search of grubs, and how to treat crabgrass and arborvitae

Linn County Master Gardener, Claire Smith, wrote the following about three of the most frequently asked questions to the Linn County Master Gardener Horticulture Hotline. The HortLine is available to answer questions from 9 am.. to noon and 1-4 p.m. Monday through Thursday  and  9 a.m. to noon on Fridays at (319)447-0647.

 

    One of the commonly asked questions in the spring concerns when to apply pre-emergent crabgrass killer.  Master Gardener Susan Long has this response:  Typically, the blooming of the forsythia or the redbud is a good indicator of when to apply pre-emergent crabgrass herbicide.  Pre-emergents must be applied before the crabgrass germinates. Ground temperatures must be a minimum of 50 degrees. If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled.  If applied too late, some crabgrass will have already germinated.  In central Iowa, this is usually mid-April to May 1.  However, if the weather warms up early or stays cool longer, then adjustments must be made based on the conditions.  Having a thick, healthy lawn that is fertilized, watered and mowed certainly discourages the growth of crabgrass. 

    Susan also answered a question about arborvitae having brown leaves due to winter burn and whether it will recover and/or should be pruned:  Avoid pruning browned, burned areas from evergreen trees and shrubs in the early spring since these branches may still have viable buds that will produce new foliage when growth resumes.  The brown will eventually fall off.  If the buds did not survive, then prune dead branches back to living tissue.  The affected trees and shrubs should look much better by late June or July.  There is no need to fertilize affected evergreens.  However, if the weather this spring is dry, periodically water evergreens to encourage new growth and speed their recovery.

    Another caller wondered what causes a lawn to be torn up at night.  Lawns that have grubs attract raccoons, skunks, and crows which turn over large patches of turf in search of the grubs.  The best time to treat is early in the summer when insecticides have the best changes of working.  The entire lawn may not need to be treated, rather treat grub “hot spots” determined by observation or sampling.  Presently trichlorfor (Dylox or Bayer 24-Hour Grub Control) and Sevin are the fastest-acting, most effective homeowner insecticides for curative grub control.  These must be watered in completely after application.  In many cases it may be preferable to repair the damage through seeding or sodding without treating.  If the old, loose sod is still green it may reattach with adequate watering.

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