Hello, Poa annua and Cardamine hirsuta have been present in my lawn every Spring for a number of years. Last Fall I decided to apply prodiamine in hopes of suppressing these weeds. It appears that the treatment has prevented the P. annua from germinating. I have not seen any. However, the C. hirsuta is just as widespread this Spring as any other. Is prodiamine ineffective against C. hirsuta? Does it germinate much earlier than P. annua making my treatment ineffective? Should I have made more than one application? In the meantime, can the C. hirsuta be eliminated with an herbicide to prevent the plants from producing seed? Thanks.
Hairy bittercress is not usually a problem in a thick lawn. Be sure to address the underlying problem. The simpliest solution is to mow higher. Most lawns are moved too low--this is a primary cause of problems. Mow around 3-3 1/2" high.
It the lawn is thin, providing too much space that weeds can use to germinate, overseed in the fall (early fall--from late August to early Sept.).
Here is info about hairy bittercress from our website: http://extension.umd.edu/hgic/hairy-bittercress You'll notice that the other big cause of this problem is too much shade, because grass needs at least 3 hours of sunlight to grow decently. If you can prune up or remove some trees to get more light, that will make a huge difference. Fine fescue grows best in shadier spots, but it does not tolerate much foot traffic or being mowed during droughts. Both will thin it, and provide space for weeds to move in. If you are dealing with a lot of shade, you might also consider replacing some turf with ground cover (avoid English ivy like the plague it is) , mulch, or ornamental beds of ferns, etc.
Control bittercress with a broadleaf postemergent herbicide when the weeds are actively growing in the spring, i.e. now. For a postemergent herbicide, look for one that lists hairy bittercress on the label.
You can also control both your weeds with a preemergent applied in early to mid-September before they germinate. However, you would not be able to sow grass seed in the fall.
Yes, Prodiamine is labed for poa annua. It is effective on grasses only, but not broadleaf weeds such as hairy bittercress. You must use a broadleaf preemergent for hairy bittercress (it has "broad" rounded leaves). But, as explained above, cultural solutions are better.
Poa annua is another weed that likes certain conditions. Here is our website info, giving you those conditions--try to change the conditions since that is easier than using herbicides over and over: http://extension.umd.edu/hgic/annual-bluegrass
You will not be able to apply an herbicide and never have either of these problems again, so it pays to figure out what conditions are encouraging them.