Lawn Weed Control: Timing is Critical

The thermometer has had some ups and downs recently, but spring is definitely here and plants are getting growing for the 2020 growing season.  Many homeowners have started their lawn management tasks for the year, whereas others have not yet had to do so due to different soil types and localized weather conditions.  One part of their lawns that most homeowners would like to do away with is weeds.

One thing that is difficult for most of us to accept is that weeds are not the problem, they are just visible symptoms of the problem.  The real problem is that the lawn is not thick enough or healthy enough to keep open soil spaces from occurring in the lawn.  Nature fills space.  In this case, it fills space with weed seeds that are given the opportunity to sprout if they get to the soil level.  The soils and climate in our area are not conducive to perfectly lush, thick lawns that will keep most weed seeds out, and thus the reason for this article.  If you constantly have issues with certain spots, it may be due to shade or soil factors.  You may want to consider conducting a soil test of the soil in that area to see if you have a significant pH imbalance or potassium deficiency.

We can’t stop nature, but we can give it fewer opportunities.  Minimizing weeds starts with proper lawn management.  Mow when needed, not because of the calendar.  When you mow, leave the cutting deck as high as you can and don’t cut more than 1/3rd of the height of the lawn at any one time.  In other words, cutting 4.5 inch tall grass back to 3 inches is about right.  The reason to leave lawn as tall as possible is that turf root system are very proportionate to the above ground mass.  A lawn cut at 3 inches will have a much larger and deeper root system than one cut at 2 inches tall.  That makes a huge difference in thickness and stress overcoming ability of your turf.

Weed management approaches vary widely, as some people like plant variety in their lawn and others want to have a very uniform lawn.  Whatever your overall approach may be, one key is to make sure that any weed management additions to your lawn are done at the optimum time of the year.  The early part of the growing season is not the best time to manage most weeds.  The exception to that statement is crabgrass.

The opportune time to apply crabgrass control products is when the forsythia (branchy, yellow-flowered shrub) start blooming and that is right about now in most areas.  Their bloom time coincides with soil temperatures being correct for crabgrass to start germinating.  As an annual weed, crabgrass can be most readily controlled with the use of pre-emergent herbicides which interfere with seedling establishment and survival.  Applying such products too early (the products only work for about 5 or 6 weeks) or too late allows some seedlings to survive, probably enough to successfully set enough seed for next year.  These products are most easily applied by using a treated fertilizer product at the label specified rates on the product.

One thing to note is that if you are not controlling crabgrass, there is no real need to fertilize until later in the year, as there is plenty of naturally available nitrogen right now.  The optimum time to fertilize your lawn if you fertilize once per year is actually in mid to late September and in our area it is often beneficial to use the winterizer type of fertilizer which applies both nitrogen and potassium.  If you are a higher level lawn manager, then you will fertilize more often, usually applying between one-half and one pound of actual nitrogen per 1,000 sq. ft. of lawn, but no more than 4 total lbs. per 1,000 sq. ft. per year.

Other key weed species in our area are mostly broadleaf perennials.  These include dandelion, ground ivy (locally known as creeping Charlie), Antennaria (pussytoes), clovers, plantains, hawkweeds, and other species.  All of these species are actually most efficiently controlled by using a broadleaf targeting herbicide containing 2,4-D; mecoprop or MCPP; and dicamba in mid September.  At that time of the year, perennials are pushing energy into their crowns to survive.  They will also absorb and translocate the herbicide product into their crowns, meaning you get the best control.  One proper fall application may give you as much control as 2 or 3 spring applications.  The next best time to manage a targeted perennial weed species is when they are in full bloom, as that is a time when their energy reserves are lowest.

Physical control of newly found species or small infestations works well, but can be physically demanding.  You can also cover patches of weeds with clear plastic that is sealed on the edges.  This will kill the plants under the area and then you will need to re-seed the lawn, so is more work but may be the right approach in certain situations.  Also, there are organic weed control products available.  For example, corn gluten meal can help manage crabgrass, but only works well after it has been used multiple years in a row and it will also inhibit turfgrass seed success dramatically if you try to re-seed an area.

One way to get more details about lawn management is to read the lawn care publications provided free on-line by UW-Madison, Division of Extension.  Links and descriptions of all these publications can be found at:  https://hort.extension.wisc.edu/topics/lawns/

Lastly, you are always welcome to contact Reuss with any horticultural or agricultural questions through phone or e-mail.  He can be reached at 715-732-7510 or e-mail to scott.reuss@wisc.edu  You can also review localized horticulture resources at:  https://marinette.extension.wisc.edu/horticulture/