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I have a strip of dwarf mondo grass down the middle of my driveway. Pennywort has started to grow in amongst it, and is gradually spreading and taking over.

I have tried manually weeding it out (was spending 10 or 15 minutes each day), however it's coming back as fast as I can remove it. Because of the mondo grass roots, it's very difficult to get all the pennywort roots out.

The mondo grass is growing quite well in areas the pennywort hasn't reached yet.

Is there anything I can do? Can I change the conditions to suit the mondo grass more than the pennywort and give it a better chance?

I don't mind using a herbicide if required. I'm not much of a gardener so any help is appreciated.

Pennywort in mondo grass

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when you are removing by hand, are you doing so after a good downpour or good soaking with the garden hose? Sorry if this is teaching you to suck eggs, but pulling weeds cleanly out of the ground is a lot easier when the soil is nice & wet. –  Mike Perry Jul 16 '11 at 4:49
    
Also how do you feel about the use of selective herbicides or do you favour and organic approach? –  Mike Perry Jul 16 '11 at 4:51
    
If it helps others, this is one of two weeds known as 'Dollar Weed'. It is creeping with thin spindly side shoots. For physical removal you have to get these and their tiny root clumps. Lawn weedkiller might be worth a try but I've faved the question as I am interested in an effective answer. –  winwaed Jul 16 '11 at 13:24
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1 Answer

Q. Can I change the conditions to suit the mondo grass more than the pennywort and give it a better chance?

A. Mondo grass (is an Ornamental grass, this is important for the herbicide part of the answer), prefers a moist soil. Pennywort thrives in wet soils. Therefore if you can reduce the moisture level in your soil you will (to some extent) hit negatively the Pennywort and improve the conditions for the Mondo grass.

  • Improve drainage by amending the soil appropriately. Example, usually that involves adding organic matter to your soil - stuff like compost, finely shredded Autumn "Fall" leaves, well rotted farmyard manure, etc. If your soil is more like clay, adding some coarse sand and fine crushed rock can be worthwhile (along with the organic matter)..
  • Water less (obviously you can't turn off the rain).

Q. I don't mind using a herbicide if required. I'm not much of a gardener so any help is appreciated.

A. From what I have read (links below), an "imazaquin" based selective herbicide can be used around some Ornamental grasses to help control/fight Pennywort. Of course before using such a selective herbicide you would need to read the label (or contact the manufacturer directly) to see if it's safe to use around Mondo grass.

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+1 I'd also add that it would be useful (even after reading the label) to spot-test the herbicide to make sure it doesn't injure the Mondo and that it kills the Pennywort. –  bstpierre Jul 16 '11 at 21:47
    
+1 For a good (safe) gardening practice tip. –  Mike Perry Jul 16 '11 at 22:19
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After about 3 weeks of no rainfall (and no watering) the pennywort was starting to shrivel. Perhaps it may have died, but now it's raining again. It appears more sensitive to lack of water than the mondo grass so will try to kill it that way as we move into summer here. –  WW01 Oct 2 '11 at 8:41
    
@WW01 Thanks for the update & good luck controlling (removing) the Pennywort via lack of water (moisture). –  Mike Perry Oct 2 '11 at 13:55
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