Questions on: Yarrow
Ron Smith, Horticulturist, NDSU Extension Service
Q: Which perennials should be cut to ground level in the fall? Specifically tall yarrow, purple coneflower, mums, and garden phlox. (E-mail reference)
A: The guidelines depend on where you live. In North Dakota, we like to let perennials remain with the flower stalks to help trap snow. Cut them back in the spring before new growth begins. I suppose in the more southern regions of the country trapping snow is not a concern while a neat looking perennial flower bed is, so cutting back in the fall would be appropriate.
Q: I would like to know when to move my yarrow, poker primrose and hosta. Also, my dirt got hard from watering all summer. Can I put new dirt over the old and work it in next to my perennials? Would it hurt to put a small amount of that dirt on top of the perennials using it as a mulch? Would you know why some of my astilbes don't get plumes? I have them on the north side of my house. I would move them if you think they’re not getting enough sun. When would it be best to move them? (Onaka, N.D.)
A: A good rule of thumb is to transplant spring-flowering perennials in the fall and fall- flowering perennials in the spring. Never layer different soil types if at all possible. Improve the tilth by adding organic matter like sphagnum peat moss. The astilbe probably didn't flower because of low light intensity or it was planted too deep. You can move it in the fall but be sure to water in well and mulch all that you transplant this fall.
Would you please tell me what kind of weed kill to use--preferably not to kill the grass and when would be the best time of year to use it. Thank you. (Devils Lake, N.D.)
A. Your fern type plant is yarrow, Achillea millefolium, an aggressive, invasive, obnoxious, persistent, deep-rooted perennial. About the only saving grace it has is that most wildlife--deer, voles, rabbits--will not eat it!
You need to pull the crowns where they have grown into your tulips, hedges or other desirable plants. In lawns, repeated applications of Trimec will bring it under control. Apply some at this time and again in early September on any survivors.
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