Controlling Japanese Stiltgrass in Your Garden

Description

Japanese stiltgrass (Microstegium vimineum) is bright green with a distinctive silvery stripe of reflective hairs along the midrib of each narrow, lance-shaped blade. The leaves are one to three inches long and alternately arranged on a branched stalk resembling a smaller, more delicate version of bamboo. Flower spikes grow on top of each stem before the plant sets seed in late August to September. Growing one to three feet tall, Japanese stiltgrass has a sprawling, mat-like manner. At the end of the season, the plant dies back; in heavily infested areas it produces a thick layer of dried, tannish thatch.

Range

Native to Asia, Japanese stiltgrass was introduced to the U.S. about one hundred years ago, probably as packing material in shipments from China. It has colonized most of the eastern United States plus Oklahoma and Texas. As well as along roads and ditches and in moist woodlands, it is increasingly found on the edges of farm fields, in pasture, hay fields, home landscapes, and turf areas.

Growth Habit

This herbaceous, annual grass germinates in the spring and grows slowly through the summer months. Its root system is shallow and weak. It tolerates full sun to heavy shade. Stiltgrass spreads through a high production of seeds and also by sprouting new shoots from the stems that come in contact with the ground. A single plant may produce between 100 and 1,000 seeds that can remain viable in the soil for at least three years. Stiltgrass thrives in areas subject to regular soil disturbances such as flooding, mowing, tilling, and high foot traffic. The small seeds may be carried by animal fur, water during heavy rains, contaminated hay, potted plants, or soil and mud stuck to footwear.

Impact on the Environment

The rapid growth and abundant seed production of Japanese stiltgrass allows it to displace less competitive native vegetation in a wide range of ecosystems. It can inhibit tree survival and growth when it changes soil nutrient cycling processes; a potential threat to the future of our forests.  White-tail deer, horses, and goats avoid it because it has little to no value as food for grazers. The deer consume native plants instead, allowing stiltgrass to invade the spaces they create. In the fall, the thick layer of smothering thatch is slow to decompose. Left unchecked, Japanese stiltgrass can overtake native vegetation in three to five years.

Prevention and Control

1. First steps

It is important to recognize the threat of this invasive plant, as outlined above, then to correctly identify it before applying any control. There are native perennial look-a-likes: Virginia cutgrass (Leersia virginica) and some knotweeds (Persicaria spp.) for example. Also, Pennsylvania smartweed (Polygonum pensylvanicum), crabgrass (Digirtaria spp.), and nimblewill (Muhlenbergia schreberi) may be confused with stiltgrass. Virginia cutgrass has longer leaves and flat compressed seed heads. Smartweed has a pink calyx and glossy black nutlets. Crabgrass and nimblewill have stalky stems and branching seed heads. None of them have the unique line of silvery hairs found on the midrib of stiltgrass. When trying to recognize stiltgrass on your property, if you are not sure, email a photograph or take a specimen to your local Extension office for identification.

2. Prevention

The objective is to prevent seed production, therefore if possible you should not wait until the infestation has spread out of bounds before using control methods. It is much easier to eradicate a small area of stiltgrass.

3. Cultural control

Mulch the soil in your ornamental beds to exclude light and stop stiltgrass from germinating. Maintain a healthy, dense lawn that competes with and prevents the establishment of weeds. If you mow your lawn too short and too often it will be more prone to stiltgrass infestation.

4. Mechanical control

Small amounts in your ornamental beds are easy to hand pull because of the plant's shallow root system. Use an appropriate weeding tool on young plants in lawns. You may use a trimmer, or a mower set very low, on larger areas of stiltgrass. Timing is very important for hand pulling, trimming, and mowing, however, because when performed early in the summer they encourage flowering and early seed dispersal. Complete these tasks in late summer when the plants are just about to flower. This way, you prevent seeds from maturing. Because the seeds remain viable in the seed bank, you will need to employ monitoring and management techniques for a number of years.

5. Chemical control

If you feel an herbicide is necessary, the home gardener should consult and hire a landscape professional who may use one of the following pre-emergence or post-emergence herbicides:   

• Pre-emergence herbicides: Pendimethalin ('Pendulum'), imazapic ('Panoramic'), and sulfometuron ('Oust XP') are effective against stiltgrass. 'Pendulum', for example, will prevent stiltgrass establishment when applied two to three weeks prior to germination, but has little effect on plants that are already present. Rainfall or irrigation is needed to dissolve the herbicide.

• Post-emergence herbicides: Post-emergence herbicides are applied directly to the growing plant and may injure all treated vegetation. Your landscape professional may use glyphosate (AquaNeat), glufosinate ('Finale'), or quizalofop ('Assure 11') to eradicate stiltgrass. The herbicide quizalofop only injures grasses; just a small amount is needed for stiltgrass control.

In conclusion, it is best to treat small infestations of stiltgrass before it spreads by simply pulling or cutting. These two methods are most effective when you wait until later in the summer and finish before the seedheads emerge. I find the stiltgrass easiest to pull when the soil is moist. When cutting, slice it off at ground level to prevent resprouting. Be persistent. With repeated, annual effort you can effectively control Japanese stiltgrass and continue to enjoy beautiful gardens that are unmarred by this pest.