Seed Swaps

Hairy Bittercress

01 Apr 2012
Cloudy 19°C / 66°F

Ok, so this is more of a lawn article than a garden article, but this year almost everyone I know in Maryland is having a mass invasion of hairy bittercress in their yard.

Here it’s in my lawn, flowerbeds, garden, planter boxes, it’s just absolutely everywhere. Since this spread from my lawn to my gardens, I figure a garden article is warranted.

I believe the mass invasion is due to the mild, rainy fall/winter we had this year.

Bittercress is a bit of a “season-backwards” weed. As a winter annual, it normally sprouts in fall and grows through the winter, and blasts out seeds in spring that lay dormant through the hot weather. This is the opposite of most plants, which thrive in summer and die or go dormant in winter. I suspect that normally the amount of bittercress here in Maryland is normally partially regulated by early onset of fall cold, and hard winter frosts and generally dryish conditions in winter.

Fortunately bittercress is highly susceptible to most lawn-care weed control products. It’s susceptible to many popular lawn herbicides including 2,4-d, triclopyr, dicamba, and mcpp (mecoprop) and most weed control products have at least one, if not 3 of these in them.

I’m not a big fan of 2,4-d and try to limit my use of it where possible, but finding weed products without it can be difficult as this is in almost every weed killer on the shelves, with a few exceptions. Environmentally, I favor triclopyr as it is much less mobile in most soil conditions (fewer contamination problems), but be careful with anything containing it, as it is more toxic than most lawn weedkillers to shrubs and small trees.

Those going organic can use corn gluten against it, but you need to apply this in the fall before it sprouts, as it is only effective against actively sprouting seeds. (several non-organic crabgrass pre-emergents including dithiopyr work against it too, but you’d also apply in fall before it sprouts, instead of spring)

If you’re having trouble controlling bittercress with weed-b-gone type products, some general tips to get the best effect from what apply (and hopefully need to apply less):

-Bittercress is most noticeable when it is still cold out in early spring, but try to spray it on a warmer day to get better absorption, colder weather closes the leaf stomata and reduces absorption. Bittercress is a bit less sensitive to cold than most, but for maximal effect, warmer is better. Cold-weather applications do still work against this plant, but take a long time to take effect. I applied some early this year, and it did kill the stuff, but it took a month to really kick in.

-Bittercress is best controlled when it first sprouts in fall. Spraying it in spring isn’t entirely pointless as it will kill them down sooner than the heat will, but the plants will likely seed anyway, so you’ll need to spray them in fall when those crop up. All you with a bittercress problem take note, if you had a lot this spring, check for it sprouting in fall (it won’t have the tall spires on it in fall, so look for the leaf clumps laying low in the lawn).

-Try not to spray it when the soil is dry, as the stomata of the plant leaves will be closed to retain moisture.

-A more humid day will keep the spray from drying out quickly, increasing plant abosrption.

Most labels will also give you these tips:
-Don’t apply when it’s windy, it will blow the spray all over and you’ll probably kill or damage some of your ornamental plants.

-Don’t apply when it’s about to rain. Even if the product says rain-proof in 3 hours, you’ll get better effect if it stays on the leaves for at least 12 hours.

So, in a perfect world, apply it on a calm, humid morning over 50 degrees F with rising temperatures, when the soil is still damp from a rain in the past day or two, but the leaves are mostly dry and no rain is forecast for the day of application.

Good luck finding a perfect day, odds are you won’t, but it always helps to know how to get more of the herbicide into the target plant, and less in the soil.




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