Landscape and Turf - Weeds of Former Farmland Sites Being Developed

Most new developments are being built on sites that were formerly farmland. The weed species that infest these new sites may be different than those in more mature landscapes. After farming ceases, sites are often left vacant to grow up in weeds before construction begins. The following is information on two weeds common in this situation.

A very common weed in this situation is marestail (Conza canadensis), also known as horseweed, a winter annual that overwinters as a rosette type of growth and then sends up a large stem and flowerstalk during the following summer. Marestail produces large amounts of light fluffy seed and can travel long distances. It also is common in un-mowed roadsides and waste areas. Mowing is effective in controlling the stem phase of marestail but not the rosette phase. Herbicides that control marestail in turf include 2,4-D, clopyralid (Lontrel) and others in the growth regulator category. In landscape beds, clopyralid (Lontrel) can be used in some situations for control, isoxaben (Gallery) has preemergence activity, and herbicides containing oxyfluorfen (Goal, Rout, OH2) also provide good control. An important issue to note is that there are glyphosate resistant strains of marestail in Delaware so Roundup and similar products may give little or no control in some cases. Canada thistle (Cirsium arvense), a noxious weed regulated by the Delaware Department of Agriculture, can also build up in these vacant sites. It is a perennial weed that can spread by the light, fluffy seed produced in late spring or early summer, as well as by underground rhizomes. Glyphosate spot sprayed is most effective in controlling Canada thistle in the fall or when it is in bud (late May or early June). Glyphosate is relatively ineffective when Canada thistle is growing vegetatively in the spring or after regrowth from rhizomes in summer. Triclopyr containing herbicides are highly effective on Canada thistle in turf (Turflon, Confront, others).

Gordon Johnson, Extension Horticulture Agent, UD, Kent County