Washington Post gardening columnist Adrian Higgins recently took questions in an online chat. Here is an edited excerpt.
Q: Do you have a good trick for establishing a ground-cover bed? What is the best way to get these tiny plants to take root, and do you have a favorite ground-cover plant?
A: Ground covers come in many forms, but when they’re effectively planted, they have the capacity to transform a landscape into something special and sublime, because they create blocks of texture and form. They’re much better than continual blankets of mulch. You should know whether your chosen ground cover is a clumper or a runner; the latter can be planted further apart and will knit together. Pay attention to proper spacing. I find a mattock is a great tool for planting little plugs, especially in heavy soil. You will need to keep the weeds back for a year or two, so in this case, mulch will be necessary. One option is to lay and peg burlap first (especially on slopes) and plant your plugs through it, making an X with a sharp knife.
Q: We have a few spots in the garden where vines spring up and try to strangle our other plants. For some reason, it’s hard to get to the roots to dig them up, so we keep cutting them back. Any ideas on how to eradicate them?
A: If you have entrenched vines, this is one of the rare instances where I would reach for an herbicide. You can apply a weedkiller to the foliage, but a spray may cause unwanted drift and plant death. One option is to wipe the herbicide on with a paintbrush. It may need two or three applications, and systemic herbicides are more effective when applied in late summer. Another option is to cut the vine a few inches above the ground and paint the wounded stem with herbicide. Check the label for your targeted plant and correct concentrations. Look out for poison ivy.
Q: We had to remove a huge maple from the edge of our patio, so we have lost all the shade there. My husband insists we can plant another as close as possible to the site of the old one. (The trunk was more than a yard across.) I say the roots of that tree, and of two other maples within 10 feet of the old one, will prevent any tree from growing in that spot. What do you say?
A: Maple roots are notoriously thick and shallow, so you may want to get someone to grind it out with a stump grinder. Determine whether you have any underground lines in the vicinity. Otherwise, you can create planting holes with a sharp ax and mattock, and your new tree should expand its root system as the old tree’s roots decay.
Q: You had a column about how climate change has affected local gardens, and how plants that used to thrive in certain areas no longer thrive. I would love to see a list or get more information about plants that are now easier to plant here and plants that no longer thrive. Do you have something like this?
A: It would be nice to have a reliable list. The difficulty with our shifting climate is that it’s a messy change. Winters are getting shorter and milder, but we can still get deep freezes that will kill plants on the margins of our hardiness zone. We are officially in Zone 7a in D.C., but I would now be comfortable planting what’s hardy to zones 7b or 8a.
Q: My first year of backyard gardening produced baskets of tomatoes, but that hasn’t happened in the 10 years since. Is there a secret to happy and productive tomato plants, or is it just luck?
A: Plant pests and diseases can build up or soil can get depleted, so initial feasts can turn to famine. Prepare an enriched bed where you haven’t grown tomatoes and peppers before, and try again. It must be a sunny site. I add eggshells and kelp meal to my planting holes and set the transplants deeper than where they were in the pot to promote more rooting. You may have to remove some lower leaves to do this. Generally, the smaller the fruit, the more vigorous and fruitful the tomato vine.
Q: I have a climbing rose bush I would like to move, but I always forget until midsummer. How do I move it?
A: Don’t move it in the middle of summer. The best time is in February or March, after you have trimmed it back and it’s sleeping. Old roses set deep roots. One option is to cut it back fairly hard in September and replant it then, so it can grow new roots before the winter. It may take a couple of years of annual pruning and tying to get it back to where you want it. You will have to protect yourself from the thorns; don’t tackle this without the proper gear.
Q: What type of fertilizer is best for run-of-the-mill annuals (Persian shield, coleus) in a container garden? I’ve heard not to use dry fertilizers in containers. Are they even needed?
A: Gardeners usually add a slow-release granular fertilizer, such as Osmocote, at planting time. A lot of potting mixes, somewhat annoyingly, contain synthetic fertilizers, whether you want them or not. I would use a slow-release fertilizer, then give a liquid feed later in the summer to perk things up (along with a trimming back).