Ask the Gardener: How to kill butterfly-murdering black swallow-wort

Ask the Expert Gardening
The black swallow-wort lures butterflies to lay their eggs on it, where they die. Jennifer Forman Orth/National Park Service

What to do this week: Plantings seldom get the weekly inch of rain they need in July and August. An inexpensive rain gauge will measure rainfall so you know how much of a deficit you need to make up. Pull played-out spring vegetable crops such as bolting lettuce, and use the space to sow fall crops of beet, bean, rutabaga, turnip, kale, collard, cabbage, and lettuce seeds directly into the garden. Continue staking tomatoes, and remove any suckers growing between the main stem and a branch. Pick cucumbers before they turn yellow. Harvest eggplants when they reach 4 inches and garlic bulbs when the top leaves start to brown. Pick blueberries in the morning, but don’t wash them until you use them.

Q. Each year black swallow-wort sprouts along the perimeter of my lawn like a perennial garden from Hades. I have not been able to eradicate it by chemical means or by pulling it or by cutting it. I happen to have a quantity of rubber roof membrane material. Do you think that it would be effective against the weed regrowing if it was laid down so none of the dirt/earth was exposed?

R.G., Medford

A. Great idea! Many experts recommend weatherproof roofing liner to smother invasives such as black swallow-wort and its cousin pink swallow-wort. Both have narrow leaves and vines that can grow up to 8 feet. Cut down the infestation and cover the area for several years, disguised with a layer of bark mulch. If you spray the still-moist sap within seconds of cutting or mowing, it helps. Another good time to spray is when the tiny eggplant purple star flowers are blooming in June, and then a second time in August. Frequent mowing will control but not eradicate it. Never let it go to seed! Dispose of cuttings in the garbage, not in leaf bags. Bad invasives are harder to kill than Rasputin and will require years of follow-up. Smothering is the best organic option. Incidentally, fans of monarchs hate this milkweed relative because it lures the butterflies to lay their eggs on it, where they die. It is totally evil.

Q. I am hoping you can advise me on pruning. I can’t seem to keep these hydrangeas straight! I want to shape old hydrangeas that are just getting too big. How and when should I do this? These are most likely Nikko and some are lace-capped.

J.F., Sandwich

A. Old-fashioned blue hydrangeas such as Nikko Blue bloom well on Cape Cod, but not in colder areas because their flower buds form the previous year and can die where winters are too cold. So prune Nikko Blue when they finish blooming in August, which is just before the new flower buds start to form. The newer blue hydrangea varieties such as Endless Summer can produce flower buds the same year, so they can be grown in colder regions. Prune these only in May after the buds that survived the winter are visible.

If your shrub is still too large after its annual pruning, you can trim it back to just above the new flower buds at the end of the summer. You can reduce a bit of the shrub’s width anytime by clipping one third of the weaker stems back to ground level to rejuvenate it. Still too big? Move it. And while you’re at it, you can divide and multiply it. Here’s a secret nurseries don’t want you to know: Most small shrubs that have lots of stems growing directly from the ground can be divided like perennials! One root ball can yield enough baby shrubs to plant a whole hedge. Wait for cooler weather and then pull or cut apart sections of roots that remain attached to stems and replant them immediately in a site that gives them plenty of room to grow to their mature size (6 by 6 feet) and enough sunlight to produce blooms.

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