Saturday, February 13, 2016 By: Ask A Master Gardener

Use Preemergent Herbicide now to prevent Crabgrass

Now is the time to use preemergent herbicides

Bill Sevier: Ask a Master Gardener

Saturday, February 13, 2016

Q: When is it time to use a preemergent herbicide? J.T., Tulsa
A: Preemergent herbicides are helpful in preventing weed establishment. They are used in early spring (mid-February through mid-March) to prevent crabgrass and other summer weeds. They also are useful in the fall (mid-August through mid-September) to prevent winter weeds such as henbit. Some of the herbicides will need a second application in spring 60 days after the first for complete coverage of crabgrass. The label on the product will indicate if this is needed.
Many people are reluctant to use herbicides of any sort for weed control. That is a reasonable choice for those who are willing to tolerate some weeds. If you have a well-established lawn, this may prevent much of the weed invasion. A healthy thick lawn depends on good soil, proper turfgrass for the area, adequate sunlight and supplemental irrigation. Most lawns need some fertilizer, and there are organic and synthetic sources available for nutrients.
Helpful lawn maintenance calendars for what to do, what to use and when to do it for Bermuda and fescue lawns are available in the turf section of the Master Gardener website.
Master Gardeners are often asked if there are any “organic” preemergent herbicides, as opposed to commercial or “synthetic” ones. Unfortunately, while there are other organic pesticides, there is no effective organic preemergent herbicide.
Corn gluten is an organic sold as crabgrass prevention. While some reports say that if applied during a narrow window in spring, there may be some benefit; OSU turfgrass specialists cite studies that show little benefit.
For those wishing to use a synthetic preemergent herbicide, OSU has some recommendations. While there are several varieties of preemergents available on the market to prevent weeds, especially crabgrass, OSU feels that one of the many commercial brands containing either the chemicals dithiopyr, pendimethalin or prodiamine are good choices. These preemergents cost a bit more than other types but last a lot longer and, in many cases, can kill crabgrass and other weeds after they have sprouted.
The labeled directions of these products must be followed. These herbicides usually come on a dry particle such as fertilizer or other inert material. They may also be found less often as liquids. They must be washed onto the soil with at least ½ inch of water after application. After washed onto the soil, they form a barrier for weed prevention, which may last for months if undisturbed.
One of the benefits of these three products is that they are not soluble in water and do not leach into groundwater or spread from where they are applied. They are broken down in nature by sunlight and soil microorganisms.

For more information or to ask a question about gardening, contact the Master Gardeners at 918-746-3701 from 9 a.m.-4 p.m.

Garden tips

§  Most bare-rooted trees and shrubs should be planted in February or March. The roots of these plants are easily damaged and should never be left exposed to air. Plant them at the same depth as in the nursery and make sure good root and soil contact is made by gentle tamping and irrigation after planting.
§  Finish pruning shade trees, summer flowering shrubs and hedges. Spring-blooming shrubs such as forsythia and azaleas may be pruned immediately after flowering. Do not top trees or prune just for the sake of pruning.
§  Dormant oil can still be applied to control overwintering insects.


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