What a weedy start to summer! People are wondering what those tall yellow-flowered weeds are that cover yards, fields, ditches and any other open space. The two most prevalent weeds we have been seeing are flixweed, Descurainia sophia, and tumble mustard, Sisymbrium altissimum. Although these are not noxious weeds defined by Nevada law, they sure are obnoxious.
Flixweed is a winter annual. It germinates early in the year and goes to seed by summer. It is a member of the mustard family from which so many of our weeds come from. It has finely dissected leaves, which differentiates from other mustards. It spreads by seed laid down in early to late summer, primarily on disturbed sites. It starts out as a rosette close to the ground and then sends up a single tall flower stalk growing 8 inches to 24 inches tall. The good thing about this weed is that it pulls out quite easily.
Flixweed’s cousin, tumble mustard is another annual, but it can grow 2 to 5 feet tall. It has much coarser, broader leaves, with many branches and has a bushy appearance. When it matures, it breaks off at the base and tumbles along spreading seed in the wind.
Although these weeds don’t do the damage to the environment, water quality and habitat that their noxious cousins tall whitetop and hoary cress do, when they dry out at the end of their growing cycle, they become a fire hazard. Besides, they make a nicely tended landscape look messy. So, what do you do to control them? It’s too late to spray because the plants are on their way to making seed. Even if you did spray, then you just have standing dead plants, which still have to be cut down. Why waste the money on herbicides? Pulling these weeds out is pretty easy. Cutting them down works, but depending on their growing stage, they may return and need to be whacked down again.
The active ingredient 2,4-Dinitrophenylhydrazine (commonly known as 2,4-D) is the only chemical listed in the Pacific Northwest Weed Management Handbook with good post-emergent (while growing) control for both these weeds. Otherwise, a number of preemergent controls are possible. These are chemicals that have to be applied to the soil prior to germination, which may require fall to early winter application, so not effective if applied now. Not all preemergents work on all weeds, so read the label to see if mustards are controlled.