Time to head off invasive weed, garlic mustard, and divide perennials

By John Fulton         

[APRIL 18, 2006]  A new weed problem has reared its ugly head in the Logan County area. Garlic mustard is making its presence known in many wooded, or formerly wooded, areas. No, garlic mustard is not exactly new, but it is expanding its range at a very speedy rate.

Garlic mustard is considered an invasive species, and some states have declared it a noxious weed. Illinois hasn't declared it such, at least not yet. The problem with garlic mustard is how quickly it spreads. It spreads so quickly it tends to choke out much of the desirable undergrowth in timber areas.

Garlic mustard is a cool-season biennial herb with stalked, triangular to heart-shaped, coarsely toothed leaves that give off an odor of garlic when crushed. First-year plants appear as a rosette of green leaves close to the ground. Rosettes remain green through the winter and develop into mature flowering plants the following spring. Flowering plants of garlic mustard reach from 2 to 3 1/2 feet in height and produce buttonlike clusters of small white flowers, each with four petals in the shape of a cross. 

Control of garlic mustard is somewhat difficult. Seeds can remain viable for at least five years in the soil. Small amounts can be pulled up (including the roots). Garlic mustard can regrow from root material. With herbicides, glyphosate (Roundup) is the most often recommended. Remember, glyphosate kills broadleaves and grasses it gets on. There has been some success with 2,4-D LV400 where there aren't concerns with other understory plants. Very large patches have been controlled with fire, but that completely destroys the understory of timbers. Remember to monitor areas for at least five years due to the seed dormancy period.

Some action at this early time may help prevent larger problems down the road. It is much easier to control small patches than large ones.

Dividing perennials

A common maintenance chore in a perennial garden is that of dividing. There is no set rule as to when to divide perennials. Some may need division every three to five years, some eight to 10 years, and some would rather you not bother them at all.

Perennials will send signals to let you know that they would like to be divided. The signals to watch out for include these: flowering reduced, with the flowers getting smaller; growth in the center of the plant dies out, leaving a hole with all the growth around the edges; plant loses vigor; plant starts to flop or open up, needing staking; or it just may have outgrown its bounds. These are the signs to look for and not a date on the calendar.

If division is indicated, spring is the preferred time to divide. Some fleshy-rooted perennials, such as poppy, peony and iris, are best divided in the late summer to very early fall.

Division is usually started when growth resumes in the spring. The process starts by digging around the plant and then lifting the entire clump out of the ground. Then, using a spade or sharp knife, cut the clump up so that each portion is the size of a quart- or gallon-sized perennial.

Discard the old, dead center and trim off any damaged roots. The divisions should be kept moist and shaded while you prepare the new planting site. After replanting, water well and protect the divisions from drying out.

Division is no more complicated than this. Some perennials may be more difficult to divide than others because of their very tenacious root system. Division has as its primary goal the rejuvenation of the perennial planting so it can continue to perform the way it was intended. Many home gardeners have found that the process of division is more traumatic to them, the gardener, than it is to the perennial.

[John Fulton, unit leader, University of Illinois Extension, Logan County Unit]