Gardening tips
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Washington Post gardening columnist Adrian Higgins recently hosted a live chat on washingtonpost.com. Here is an edited excerpt.

Q: The city was kind enough to expand a tree well in front of my house, but it did not plant anything or even put in good soil. Now it is a mass of weeds. What can I do to make this area remotely plantable next spring? Should I put down newspaper, cardboard and mulch to kill all the weeds over the winter? I’m looking for a quick and easy solution this fall and will allocate more time next spring.

A: The first thing I would do is pull the weeds and then put down a thick mulch. Layers of newspaper and cardboard will help block the weeds and weed seeds already in the soil, but not the weed seeds that will continue to arrive and germinate in your bed. These may include winter annuals that if left untended until next spring would become a right jungle.

Q: We have gotten too many slugs this year, killing many a perennial. How do I control them? We put down newspaper and mulch around our plants. Could this be our problem?

A: It’s been a fairly wet summer, and the slugs are happy. You can use organic slug killer, but I’d be leery of that in and around edible plants. Beer traps are effective, if you police them. You can also go out at night with a flashlight and put the slugs in a pail of bleach solution, but use gloves. The slime is pretty awful and persistent.

Q: We have a beautiful oakleaf hydrangea as a foundation planting, but it’s overtaking the yard. When, how and how far should we cut it back?

A: They don’t lend themselves to easy pruning. If they are too large, you can remove whole stems, but do it without disfiguring the shrub. Removing them now may lose you blooms next year.

Q: What’s your favorite dwarf crape myrtle for this area?

A: The National Arboretum has been a leader in the release of mildew-resistant crape myrtles. A truly dwarf introduction is named Pocomoke. I also planted a cherry-red cultivar about eight years ago (named Cherry Dazzle), and it has grown to about three feet high and four feet across, bigger than anticipated. They can be trimmed back or even treated as a perennial by cutting them back hard in winter, but only after they are established.

Q: I have three red-twig dogwoods (“Cream Cracker”) along the foundation of the front of my house under a large window. They were planted three years ago and looked great this spring. Now they don’t look well and have black spots on their stems and leaves. I gather this is a fungus. If this is the case, are they salvageable? If not, what would you recommend replacing them with? I have always thought they were planted too close to the house and are too crowded.

A: If I lived in, say, rural Vermont or Scotland, I would plant oodles of red-twig dogwood, especially on wet land. A massing can look spectacular in winter, when the twigs glow brightly. They underperform in hot, humid climates; their heart doesn’t seem to be in it. They can look OK with proper care, which includes giving them space, ample moisture and an annual regime of rejuvenative pruning, but otherwise they’re a bit of a letdown. They suffer from a canker disease when stressed. You could try to revive yours by cutting out the diseased stems, giving them some watering (but not feeding at this point in the year) and a nice mulch of leaf mold, and then removing all the oldest canes in March. If you wanted to cut your losses and plant something else, I do like oakleaf hydrangea as a shrub about the same size but with more seasonal interest.

Q: Is there a showy perennial or flowering shrub for damp shade that I can put in with a river birch this autumn? I’ve planted a lot of natives recently, so feel free to splurge without that constraint here.

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A: To me, birch groves cry out for something a little restrained, perhaps with some ferns or, again, sedges (there are many that will take damp conditions). One quirky thing that might do the trick is the hardy begonia, which spreads when happy in damp areas and forms colonies of late-season perennials. You could add Japanese or Siberian irises for spring display.

Q: I have three unappealing arborvitae. I want to replace them, preferably with something that provides good bird habitat without growing too wide. Have any good options for me?

A: I might try something more shrubby, which will give birds a place to roost and get away from predators. Is Burford holly too trite? You could try yaupon or American hollies or larger forms of false cypress.

Q: What can I plant now? I’m itching to put something in the ground. I have a 5-by-10-foot planter that’s a blank slate.

A: My general advice is don’t plant a big tree or shrub that in five years you will be constantly chopping back. Nor would I plant a lot of fussy things. I would pick three varieties of perennials and perhaps a small shrub and do some massed plantings that will look handsome and calm.

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