Q: Yarrow is taking over my wildflower bed. The bed is quite large and contains many varieties of wildflowers. But just like last year, the yarrow is running rampant this spring. My attempts at controlling - digging and pulling roots - don't seem to be enough. Is there something I can do to at least control the spread?
A: Another case of a good flower gone bad. Isn't that always the case? When we want a plant to spread and fill in to cover a large area, it takes forever. But when we want plants to stay put, they take off. Sometimes the same plant in a neighbor's garden will stay put for years or even die.
The first thing to do when you have a bad plant is to find out how the rogue is spreading. Some plants spread by seeds and some by horizontal growth of stems or roots, either above or below ground. Are the new plants small seedlings with their own roots, or do roots connect them horizontally?
When first planted, plants often behave themselves for a year or two before causing problems. It is building up strength. If it spreads by seeds, it may take time to mature or grow large enough to produce enough flowers and seeds to cause a noticeable problem.
For seeding plants, if you cut off the flowers before they produce seeds, it will limit spreading in the future. You can also apply a pre-emergent weed stopper to the beds for the next couple of years to limit the existing seeds from sprouting in the future.
If the pesky plants spread by roots or rhizomes, they often take a few years it to become established and spread. At first, the new growth might look like the original clumps are getting bigger, but then they really throw out new growth and spread to cover a large area all at once.
If the plants spread by roots and rhizomes dig a trench around them to stop the spread. For yarrow, check the roots, but I think a trench the depth of a shovelhead ought to do it. If you want only to keep the original yarrow plants, then dig a trench around them. Now you can eradicate the remaining unwanted yarrow.
There are several ways to kill yarrow and other unwanted plants. You can hand dig and pull them out roots and all. Of course, missed roots could re-sprout. But if you keep after them, you will win. The key to stopping yarrow's return is persistence - week after week.
The more often you make an unwanted plants strive to re-grow, the more stress is put on their roots' stored resources. Consequently, it is unnecessary to pull out the whole root if you cut down new sprouts on a regular basis. Eventually this will kill unwanted plants, but only if you are persistent.
A long-handled tool I use for dandelions and other weeds in my lawn and perennial bed is the Precision Weeder. It has a long handle and a small metal tip that slices through roots, eliminating the need to bend over. It is good for thistles, which are nasty to touch, and it is good for harvesting asparagus. With the Precision Weeder I can stay on the flagstone path while removing weeds growing in between my good plants.
Check your local garden store for the Precision Weeder, or find it online at www.wingedweeder.com.
Another tool, the Winged Weeder, is good for large areas like between garden rows. Either tool could be a big help for your back, while you remove yarrow.
You can kill plants with scalding water. Heat water to the temperature you would use to make tea and then pour it on the plants. It will melt the wax off the leaves and dry out the plant down to the roots. However, boiling water kills only the top of the plant without killing the roots. The plant may re-grow. This method will require monitoring to see if it needs to be done again.
Finally, you can use herbicide. Either a broad-leaf weedkiller or a total plant killer will work. You must work carefully to keep herbicide from touching plants you want to keep alive. Follow label directions for best results.
Work in early evening when there is no wind. Get real close with the sprayer to the unwanted plants. Shield good plants with plastic.
You might need to follow up, but there might be so few new sprouts that you can pull them or cut them down by hand and avoid re-spraying. Spraying early in the spring is best because young plants respond better to sprays and desired plants will be smaller and farther away from the unwanted ones. A second spray later in the spring might be more difficult after desired plants have spread in size.
E-mail questions to Jeff Rugg at firstname.lastname@example.org.