More tips for taming garden bullies
Don't spoil them: Most thugs do well in poor soil with little maintenance, but if you pamper plants such as spurge or evening primrose (Oenothera spp.) with more water and fertilizer, they'll become invasive.
Pull, pull, pull: The only way to control some plants is to physically pull out new shoots, roots and all. Although hard work, relentless pulling will eventually eliminate most bullies. This is effective for controlling tawny daylilies, lambs' ears, dame's rocket (Hesperis matronalis), common violets and English ivy (Hedera helix), and for seedlings of European cranberry bush (Viburnum opulus) and Tatarian honeysuckle (Lonicera tatarica).
Mulch: Thorough mulching prevents seeds from touching bare soil. Cover the ground with a three- to five-centimetre layer of non-living material such as hay, straw, grass clippings, wood chips or black plastic film to also deprive seeds of light, which they need in order to germinate.
Relocate: Find a place in your garden where a plant's rampant tendencies are an asset. A friend of mine, for example, used to hate trumpet vine (Campsis radicans) until she put its rapidly rambling nature to good use by trailing it over an arbour. Site would-be bullies in rock gardens or on slopes, where they can help control soil erosion. Evening primrose, snow in summer (Cerastium tomentosum) and spotted deadnettle may be too aggressive for the perennial border but just right for a hot or dry-shade area where little else will survive. My sister has a lovely, well-behaved, narrow border of goutweed—perhaps the worst offender of all—with tawny daylilies growing in poor soil, surrounded by asphalt on one side and a cement wall on the other.
Contain them: Some plants should be grown in pots to contain them; herbs such as mint, thyme and marjoram are good examples. To control horizontal spreading of Artemisia—particularly western mugwort (A. ludoviciana ‘Silver King' and ‘Silver Queen') and A. stelleriana—plant it in a bottomless (for good drainage) container sunk into the ground up to rim level.
Harvest early: Pick herbs in mid-June before plants flower, and pinch off any regrowth before it has a chance to go to seed.
Reward good behaviour: Plant less invasive cultivars of thuggish species such as ‘Miss Manners', a clumping, non-spreading, white form of obedient plant (Physostegia virginiana), ‘Bressingham' soapwort or clumping bamboo, such as Fargesia spp.
Bring out the big guns: If a non-chemical solution will work for you, use it. But sometimes a chemical, such as glyphosate (marketed as Roundup or Rodeo), is the only sure way to stop aggressors such as Tatarian honeysuckle, Japanese barberry, European cranberry bush or even periwinkle. Make sure the herbicide you're buying is effective on the species you want to kill; follow directions exactly.
And finally, before you accept a gift from someone else's garden, keep in mind that if they have too many of whatever it is, chances are before long, you will too.