Thursday, September 11, 2008
Multiple factors affect weed growth in grass
John Arbogast
Landscape consultant John Arbogast answers your questions every Thursday. Send questions about your lawn, garden, plants, or insects to:
Dear John
5102 Greenfield St. SW
Roanoke, Va. 24018
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Q: Here in the eastern middle United States, our growing season is almost over, but we’d like your opinion. We have a decent fescue and bluegrass lawn out front and to one side, but we seem to have had more lawn weeds these last couple of years than we used to. We have fertilized the lawn and we did apply some lawn weed killer. The big difference I can think of is that we have had the lawn cut a little shorter than we used to. We used to keep the mower set on the medium or next notch higher cutting height, but these last couple of years the mower has been set at a lower setting. My family likes to see that the grass has been cut. Could the lower cutting height cause the weeds to be more prolific? If so, what can we do about it?
A: Yes, cutting at a shorter cutting height is known to weaken grass plants thus allowing both broadleaf weeds like dandelions and grassy weeds like crabgrass to emerge and thrive. Mowing height adversely affects root growth. Golfers might know about this from the high maintenance care that is given to golf greens that usually are composed of the grass called bentgrass that is mowed very short to facilitate putting. This close cutting weakens the bentgrass plants, so they now need lots of care, including weed and disease management in order for the weakened bentgrass plants to survive. Other site specific factors can be involved with weed growth too, but let me explain what you can do in addition to raising the cutting height now and for next year.
I have no scientific proof of this, but I personally wonder if the increase of carbon dioxide in the atmosphere is encouraging more weeds? Carbon dioxide is known to enhance plant growth.
First, consider the time to fertilize your lawn. Bluegrasses and turf-type tall fescues benefit from receiving most of their annual amount of the nutrient nitrogen in the early and mid fall months here in the temperate climate of the eastern states. At that season, the nutrient nitrogen that normally goes to new green growth actually helps the roots of bluegrasses and fescues and makes the grass plants grow thicker. Nitrogen applied in the spring, especially after the weather warms up slightly, is used readily for lush leafy green growth rather than for increased grass plant density or for root growth. A lush spring bluegrass or fescue might become a thin lawn once the weather turns hot and dry, thereby giving greater opportunity for weed growth.
Soil testing and pH correction if needed can contribute to a greater opportunity for weed well-being too. Bluegrasses and fescues get the nutrients that they need at a soil pH of around 6.2 to 6.5 Applying commercially made turf fertilizer every year tends to make the mid Atlantic soils more acidic naturally. Without taking soil tests every third year to see if the soil chemistry needs lime also can make the grass become thinner gradually giving the weeds an unintentional advantage.
How much blade height is removed per mowing is another weed enhancing question that you must ask now and again next year. If your shorter cutting height also means that you regularly remove more than one-third of the grass height per mowing, you have been causing the grass roots to become weaker or even to die.
When do you apply broadleaf weed killers? Time of application should be aimed at the typical tender time of major weed growth. Many of our nasty lawn weeds like white blooming clover and dandelions are perennial plants by nature. They should be tenderer and thus more readily killed by appropriate lawn weed killer application in October and November here in the mid-Atlantic region. Other problem weeds like lespedeza and prostrate spurge are summer annual plants that are tender and killable in April and May.
Soil compaction caused by foot traffic on dry soil crushing small soil particles into tiny pieces leaving little air space definitely weakens good grasses and assists weed growth. The remedy in our on-going drought climate change is to limit foot, lawn equipment, play vehicles, and dog paws from dry turf. Stick to the sidewalk.
Q: A friend recommended that I plant a Japanese Pagodatree next spring for a large growing tree that makes yellow flowers in midsummer and tolerates summer heat and drought after it becomes established. Is that a good recommendation? I live in the lower mountains of Virginia.
A: You were told correctly. Water the tree weekly for the first two growing seasons. The only expectation I would pass along is that the gorgeous yellow flowers might not come until a Japanese Pagodatree (botanic name is Sophora japonica) has been established for 10 or more years.
