Washington Post gardening columnist Adrian Higgins answered questions recently in an online chat. Here is an edited excerpt.
Q: What would be a good plant, maybe a ground cover, for alongside a house that is constantly in the shade? The roof runoff ,along with grading, makes the area damp and prone to puddles after heavy rains. I am exploring additional French drain installation as well.
A: I would consider some robust ferns, such as ostrich, royal or cinnamon fern — a lot of them — as well as hardy and swamp hibiscus.
Q: I’ve given up on my roses this year, but looking ahead to 2022, what is the best treatment to prevent black spot?
A: Roses need to be planted in an area where it is breezy, or at least not with stagnant air, as you might have where fences come together. You need good garden sanitation — remove all leaves as they yellow and pick up all the fallen ones, including the ones from last season.
Correct pruning to open up the center of a bush is important too, but the most effective way to counter black spot is to first buy varieties that have been specifically bred to be resistant, of which there are now many on the market. That would be my first criterion in picking a rose in a hot, humid environment.
Q: Do you have any thoughts on “lo-mow” grasses?
A: Low-mow grasses are typically used for more shady areas. They’re not as robust as turf-type tall fescues and won’t take a lot of foot traffic. If you don’t want to mow at all, you could put in some suitable ground covers or some lower-growing ornamental grasses or sedges, but installing those will require initial work and cost.
Q: Are there any perennials that do not benefit from deadheading? I have heard that phlox will produce repeat blooms quicker if it is not deadheaded.
A: Some perennials will respond to deadheading better than others, in terms of reblooming, but they are generally programmed to flower once in the year, albeit for a long time. I haven’t heard that garden phlox prefers not to be deadheaded. The idea of deadheading is to disrupt the plant’s natural life cycle by preventing seed set and thus tricking it into blooming afresh.
Q: I have two fig trees that are doing well in large pots. They are about six years old. Can they stay in pots indefinitely? Should I ever prune the roots or repot?
A: You can keep figs in pots indefinitely, provided that the container is large enough. I would keep the top growth trimmed annually to keep the top in sync with the roots, and I would consider either root pruning to keep it in the present container or potting it on to something a bit larger. If you keep the containers outside, they will have to be frost proof, and know too that growing in a container effectively puts the plant in a colder growing zone (e.g. 6b instead of 8a).
Q: What’s your recommendation — aside from constant edging — to keep my grass on my side of the property line?
A: I would consider installing a thick steel edging, maybe a 6-inch-deep one if you can find it, and turn this into an early fall project. Form a trench with a mattock and lay the edging in a way that most of it is buried. A level will keep the top edges horizontal. Displaced turf can be put back as plugs that will fill in next year.
Q: Are there any perennials or vegetables that I can start growing under lights now to be ready for fall?
A: You can start veggies now for a fall crop, either in the soil — such as carrots or radishes — or in flats — such as lettuce, kale, pak choi, etc. — that you can plant out in a month. You could start perennials now from seed, outside in a protected area would be good. An open cold frame would be ideal, where they could be left over the winter. Hardy perennials need to be in a cool environment in winter dormancy. Figure out a way to keep the squirrels at bay.
Q: I recently removed a bunch of invasive periwinkle/vinca from under a large maple tree. The area is incredibly root bound, but I’d like to put in a native ground cover or two to fill the space and prevent the vinca from returning. The tree has been limbed up so there’s a fair amount of sun, probably part shade. What would you recommend that will do OK amid the thick root mass?
A: Maple trees have some of the worst surface roots; the worst being the now much-maligned Norway maple. The problem is made worse where soils are thin and there is a lot of subsoil compaction. It is difficult to get things established in such an environment, but the key is to start small, with plugs or even seed. Small bulbs such as scilla, glory-of-the-snow and even cyclamen might get established there. Another option is to grow things in containers under the tree.
Q: When is the best time to propagate a rambling rose?
A: Now would be a good time to take cuttings and place them in a sand/compost mix. Keep them moist and protected and they should root over the next few months to be ready for potting next spring for planting the following fall.