<> HELP! Our lawn is being invaded by Japanese Stiltgrass! We've been told that herbicides are the best way to get rid of it, but we don't want to kill everything else in the yard. We've tried pulling it up by hand but can't seem to get it all. What should we do?
--- Jason and Tina in Patterson, New York
A. Keep pulling! Even University Extension Bulletins that praise the use of chemical herbicides note that stiltgrass is very shallow-rooted and easy to pull out by hand, especially when the soil is wet and especially especially in late summer, when the plants are tall and easy to grab. Wait until right after a heavy rain or soak the soil yourself, then reach down low and pull gently, right at the soil line; you should be able to get the plants out easily with all of the roots attached. (And bonus—late summer is also the best time of year to sow new grass seed afterwards to fill in any bare spots you create.)
Always pull your weeds when the soil is wet; it's hugely important if you want your pulling time to be well spent. Roots are easy to coax out of wet soil, but they'll just snap off if it's dry down there. And 'arranging' for wet soil shouldn't be hard with this weed, as it prefers to grow in moist, shady spots—conditions that aren't good for the lawn itself.
Take shade. Although some lawn grasses can survive with just a few hours of sun a day, they tend to be thin, delicate and totally intolerant of foot traffic, making it easy for weeds to move in. And if they're a cool-season grass (like fine fescue, the most shade tolerant of the lawn grasses), they're also going to struggle in the heat of summer, when warm season plants like stiltgrass are at their strongest.
Because stiltgrass will always have the upper hand in a wet, shady cool-season lawn, it's really important to try and improve the growing conditions as much as possible. The more sun that gets through, the stronger the desired grass will be and the less attractive the site will be to stiltgrass. So if the area has become increasingly shaded by nearby plants getting bigger over time, some judicious and correctly timed pruning to let more light in would be a big help. (Just be sure and read our previous articles on pruning BEFORE you start cutting.)
And then there's the wetness factor. Renting a machine called a core aerator to pull little plugs out of the lawn to improve the drainage will also make the area less inviting to stiltgrass. As will 'wise watering'; lawns do best when they're watered deeply and in-frequently. Frequent watering keeps the soil constantly wet, which is bad for your grass —especially when there isn't a lot of sunlight to help the grass process that water.
So here's the plan: If you have stiltgrass invading a cool season lawn, pull it out now (late summer/early fall), overseed the bare areas, improve the drainage with core aeration—which you would also do now with a cool-season grass—water wisely, and think about pruning or removing some plants that are blocking the sun. Then be ready to apply corn gluten meal as a natural pre-emergent herbicide in the Spring.
Stiltgrass is a summer annual, just like crabgrass. That means that this year's plants will die over the winter, but like crabgrass, will produce a lot of seed first. Pulling up the adult plants in late summer before they can drop that seed can be a huge help in limiting the following year's infestation. But you also want to make sure you apply corn gluten next Spring to prevent any 'missed' seeds from sprouting—just like when you're fighting crabgrass.
…With one important exception. Stiltgrass seeds can remain viable in the soil for several years, so you should apply corn gluten in the spring even if you think you pulled up every plant before it could set seed. There may well be stiltgrass seeds from previous years lurking in your soil. And you should feed a cool-season lawn in the Spring anyway, which corn gluten also does perfectly (and naturally).
Ah, but what if the stiltgrass is in a warm-season lawn? That's the problem that Rachel in North Wales, PA wrote us about. She says: "Nasty invasive stilt grass is taking over our zoysia grass, which I thought was the toughest stuff ever!"
I've often said the same thing about zoysia, and must confess that I'm very surprised that this weed is beating it! Zoysia is a warm season grass that goes tan and dormant in cold weather but stands up to summer heat much better than cool-season grasses; and since zoysia is at its strongest in summer (and a very aggressive spreader), it should be able to outcompete stiltgrass.
At any rate, the answers are going to be pretty much the same, but the timing is going to be radically different. With a warm-season grass, I would pull the stiltgrass as early in the summer as possible, while the zoysia (or Bermuda or St. Augustine…) is growing vigorously and can quickly fill in the bare spots.
And if the lawn drains poorly, core aeration is still the answer—but with a warm-season grass, those plugs should be pulled in the Spring, not in the fall.