Washington Post gardening columnist Adrian Higgins answered questions recently in an online chat. Here is an edited excerpt.
Q: Our front lawn, landscaped five years ago, has become very crowded. It has pachysandra, Lenten roses, ferns, etc. How do I make it less crowded? Could I reduce the number of plants? Is there a recommended spacing between plants of the same species?
A: Perennials that clump (instead of spread) tend to peter out after several years, although Lenten roses are not known for this and rarely need dividing. If your plants are crowded and flagging, then you should lift and divide them, and this will give you an opportunity to improve the soil when they go back. If you do this now, give them a good drink when replanted and be careful not to damage foliage.
As a ground cover, pachysandra is old hat and I would encourage you to find other ground covers to use, but to plant them in late summer/early fall. Ferns, sedges and cranesbills are among those that will freshen up these areas, but be guided by your light conditions.
Q: I saw it recommended that we remove the peony seed pods to promote more flowers the following year. I have not heard of this before, what are your thoughts? I generally cut off the spent flowers when the petals start to fall off, but is this the best practice?
A: As a general rule, removing fading flowers will prevent the plant from pouring energy into seed production, so yes, this will benefit your peonies. What may be more important is to keep them watered as they grow and bud, and a light feed at that time wouldn’t go amiss.
Q: I planted cabbage for the first — but not last — time, last year. When I harvested the cabbages I just cut off the tops and left the roots. They kept growing and now they have lots of leaves and sort of look like a broccoli stalk with stems and leaves and yellow flowers — I don’t see any heads forming. I harvested the leaves from one plant and sauteed them with olive oil and garlic, and they were basically tasteless. Is there anything I can do at this point to get a cabbage or something edible out of my remaining plant? I talked to a farmer at the farmers market and he said he always pulls the plants and replants and he had no idea what would grow. It’s starting to spread and take up a large section of my garden, so I figured I’d check with you before I pull the final plant.
A: Basically, your cabbage plants have bolted from the roots. I like that you tried them to see if they were tasty — gardening is all about one’s own experimentation and experiences. Don’t expect much from these particular plants. You could plant cabbage transplants now — I’d give them a bit of shade for a week or so to get them acclimated to the arriving summer. Keep them watered and free of cabbage worms. Chinese cabbage is easier and quicker than traditional varieties, and certainly worth planting in August for a fall crop. You can start transplants from seed indoors in early July.
Q: Which flowering annuals would do fine in a very sunny and hot border location?
A: One obvious heat loving annual (in cold climates) is the lantana; another is calibrachoa.
Q: We’re thinking of going away for a month or more this summer. Are there (nonchemical) ways to minimize weed growth while we are gone, aside from hiring someone to come by every week?
A: The best remedy against weeds is to crowd them out with other plants. Weeds fill a void. I don’t know what sort of area you are trying to protect. One issue with mass planting now, apart from abandoning young plants when they most need pampering, is that any soil disturbance will bring weed seeds to the fore. One simple — if not entirely foolproof — approach is to put down a two-inch layer of mulch just before you leave, making sure that no soil is left bare. But don’t go nuts with a ton of mulch.
Q: I have several unwieldy nandina plants in front of my porch and I’d like to replace them. What are some good alternatives for a bed that is 4 feet by 8 feet and gets partial sun? I don’t have good soil.
A: I would plant a stand of ornamental grasses of similar stature. I am a big fan of panicum, molinia and deschampsia grasses.
Q: I’m looking for a plant that can be a lawn alternative. I’ve tried mini clover that advertised a 6-inch maximum but grew to 14 inches the second year. I hate mowing and hernararia glabra is slow-growing. Do you have other ideas?
A: I’m not sure what sort of areas we are talking about but maybe you need a better lawn mower. Lawns have their place, but not as the default landscape feature for want of something else. Consider reducing the size of your lawn and developing beds of trees, shrubs and ground covers. You could spend the summer planning for the makeover and doing the soil work and then roll up your sleeves in September.
Q: I have two 15 year-old Japanese holly shrubs on the north side of my house, which is almost entirely in the shade. About five years ago they developed a sooty coating near their centers and I read that this is a result of aphids. I treated them with a mixture of vegetable oil and Palmolive dish soap in water, which seemed to contain the problem, but not cure it. I neglected to do this for the past two years and the problem got worse. One plant looks worse and the other plant is nearly dead. I want to remove them and replace them with new shrubs, possibly Japanese Skimmia. Will planting these shrubs in the same place as the infested shrubs cause them to become infested with aphids? Is there something I can do to treat the area first?
A: In fresh spring growth, hollies can attract aphids, which secrete a sugary substance named honeydew. That, in turn, becomes colonized by a fungus named sootymold. This coats the leaf with a black pigment that interferes with photosynthesis. The remedy is to do what you can to counter the aphids, either by simple squirts from the garden hose or with insecticidal soap. In theory, you can buy loads of ladybird beetles (ladybugs) for the larvae to devour the aphids, but I’m not sure I would count on their efficacy. Skimmia is a sort of old fashioned, low growing shrub, which has its appeal, but like the holly, you will need a male plant to go along with the female ones for berry set. In our changing climate, I would give it a location with some shade.