Adrian Higgins on reblooming plants, and all things gardening

By Jennifer Heffner For The Washington Post
Aug 22, 2019

Washington Post gardening columnist Adrian Higgins took questions on gardening.

Hi, Mr. Higgins. I get the impression that there are at least two varieties of crape myrtles in terms of habit - tree-like and bush-like. I was never told which applied to the three crape myrtles I planted, which have now grown tall - 15 feet or more - putting the display of flowers out of sight from the house. Is it possible (or even advisable) to prune them shorter (after flowering, I would assume), or should I let well enough alone and wish that I had planted the bush-type variety? Thanks.

Hello, everyone. I'm ready for the weather to break and to tackle garden tasks I've put off because of the heat and humidity in what has become another abnormally hot and steamy summer. Crape myrtles vary widely in habit and application and it is extremely important that people pay attention to the name of the variety they are buying or having installed. They vary from four feet high to nearly 40 feet high (crape myrtles, not people) Here, for example, is a list of just a few of the introductions from the National Arboretum.

The popular Natchez, as I've said before, grows to 30 feet or more. If your cultivar is the wrong size, i.e. too big, you can either replace it, try to move it (if it's young) or carefully and conservatively prune out some entire branches to give it a more open look. What you should not do is simply cut the tree back -- topping, it's called -- to create stubs or a hat-rack effect. This is commonly done in late winter and is appalling. There is a related practice called pollarding, in which the tree is cut back to just above the same point each year and in time it produces "knuckles." This is done to lindens and plane trees in France and England and I suppose it is one way of dealing with the problem. It is much better to have a clear sense of how large a woody plant will get after a few years. 

Producer here, sharing a link to Adrian's most recent column, on reblooming varieties of plants.

I planted some "Mammoth" sunflowers in my backyard garden and now have some blooms about 12 feet above the ground. The problem is that many of these blooms are drooping over. I also noticed that some of the lower leaves are showing fungal spots. I have cut them off, but have no idea if this is to blame for the drooping, or if it is just the hot weather. I have no idea what to do. Water more? Water less? Remove more leaves? Any insights would be appreciated since we are close to a month away from when the seeds are supposed to be ripe.

In general, the larger the sunflower the fewer the blooms. Mammoth is a tall monster and like others of its ilk, the flower head tends to nod down after pollination. Perhaps this is to protect the seeds from rain, just a guess. In my story today, you can read about Helianthus SunBelievable Brown Eyed Girl, which grows to just three feet but can produce as many as 1,000 blooms before the frost. Feel free to remove the blighted leaves on your sunflower.

Unfortunately, I neglected a very large flower bed, which is now mostly full of nutsedge, wild strawberry and all kind of common weeds. It is quite daunting - I don't know where to start. Weed whack them and cover with black plastic? I don't want to use herbicides if I can avoid it. Please help!

Weeds adore busy and distracted people. You can ignore a garden for a week in this tropical heat and you will have explosions of nutsedge, galinsoga, sorrel and many other fast growing weeds. Unless you can remove the "nuts" in the rootzone of yellow nutsedge, it will grow back. I would suggest buying a sharp weeding hoe or weeding knife, which make fast work of large areas. You don't have to weed the whole area in one go. You can pace yourself over two or three days. Just make sure the cleared earth is then mulched to avoid the weeds shooting right back. 

I lost a dogwood that had been just fine for 27 years, since I moved in to my home. Looked fine in the spring, and all of a sudden I noticed it was dead several weeks ago. It is at the bottom of a slope -- maybe too much water this year?

I think we are seeing the delayed results of last year's record rainfall. In terms of a replacement, you may want to find a treet that will take wetter conditions -- bald cypress, black gum?

Hi Adrian, Your column (8/21) on re-blooming plants has me wondering. Will these plants be providing a longer-lasting source of nectar and pollen for pollinators? And do you think the plants will be as long-lived as the once-and-done cultivars? Thanks.

Thank you for your thoughtful question. In theory, the longer  a plant flowers the more nectar it will provide. Many of these plants are sterile, the sunflower, for instance, so they don't produce pollen, but will provide nectar. I have to say that I am much more interested in plants that bloom for a long time but not season long -- I want to see a long and different parade created by planting more things in the garden.

I'm going to need some shrubs to fill in a space on the front of my house. The current ones -- so generic I don't know how to describe them -- are pretty dull. I'd like some picks that will grow to be about six feet in height and fill other spaces nearby with something colorful. Do you have any advice on favorite shrubs? A website that would allow me to scroll through some photos?

The key question is whether the site is in full sun or shade. Beyond that, I like Corylopsis and fothergillas -- both of which stay much smaller than related witch hazels. Some of the shrub-like Acers are lovely as well -- the weeping form is a bit trite.  

Around 4 years ago, I planted a then very tiny spice bush and a small Miss Kim lilac next to eachother. Too close, as it turns out. I had no idea how big spice bushes got and it is at least 7 ft tall now and crowding out the lilac! I'm assuming I am past the point where I can move the spice bush successfully, do you think I'd have better luck with the lilac? It is smaller, I am just concerned their root systems may be entwined by now.

Alas, both of these don't transplant well because of their root structures. It may be a question of picking one over the other. Given our trajectory of heat, I think I'd show Miss Kim the door. 

I'm in suburban Maryland, and most of the recent thunderstorm have missed my neighborhood, the ground is really dry. I'm watering my bushes and trees once a week with a sprinkler or a soaker hose for about two hours, but I wonder if that's enough.

When it turns dry, plants need 1 to 2 inches of water per week. It is important that you give the ground a good soaking once a week or so, rather than a sprinkling every day. You want the soil to become fully wet down to four inches, and you can test for that by plunging a screwdriver into the ground to see how far it goes. Enriched soil and mulches will retain moisture better than unimproved clay, of course. 

We planted a lilac bush mid-spring. It was doing really well, but given so much heat this summer, it now looks sad and brown. Is there a way to tell if it is maybe still alive and will revive itself next spring?

When spring comes along, our impulse is to plant. For woody plants in particular, planting in April or May can be a problem because just as they are getting over transplant shock, along comes heat stress for root systems that are inherently under-developed. Overwatering is often the cause of new plants not making it. September and October are better months for such planting.  

My emerald green arborvitae is now half brown. Should it be removed? Any idea why this might have happened?

Conifers generally don't regenerate from dead wood. I might take it out. This sounds like it could be from bagworm damage -- look for camouflaged cocoons. Bagworms are generally treated earlier in the season before they get big and do a lot of damage. It might also be root rot from wet soil. 

We recently moved into a new house that has a lovely large window box. It is currently empty, but the previous owners used it year round. Anything we should know about planting in window boxes? Is there something we could plant now for late summer/ fall that would look nice? We live right outside of DC. Thanks!

Window boxes are much more work here than in Europe, where they abound. The first problem is the use of insect screens on windows in the U.S. The bigger problem is the heat, which dries out window boxes and hanging baskets within a few hours. Generally, the greater the mass of soil the better for the plants, but such boxes get heavy with water and need to be securely anchored. On 90 degree days, they will need to be watered twice a day. 

I have to pull up 2 crepe myrtles succumbing to powdery mildew. I would like to replace with a more resistant variety. Can I plant the new crepe myrtles in same spots, or is that not a good idea?

You can find varieties bred for mildew resistance. Powdery mildew is a problem in corners of the garden where there is poor air circulation. This can be mitigated by pruning in a way that opens up the plant. But if your site is enclosed by fences, walls etc. you may want to find another plant for that location. 

Hi Adrian, My azaleas have become very large and woody with big bare patches. Is pruning effective for controlling size, or should I replace them? They used to be petite and were much prettier then.

This just in: Shrubs grow! Please don't think a plant at the garden center is going to stay the same size or anything close to it. On old, big azaleas, you can reduce the mass by trimming them back, but don't leave branch stubs. Or you can keep much of the height but take out whole branches to open up the whole structure. The key is bring an artistic eye to it, study the plant from all angles before cutting. If you think you'll botch it, don't do it.  Removing growth now will remove some of next spring's flower show. 

Interesting that the first question was about crape myrtles, as this is the time of year when my dislike of them reaches full pitch. My husband, in contrast, really likes them. Are there any varieties that don't self-seed as readily as the varieties we have (and I don't know which ones we have, but there are a at least 3 in different shades of pink). It's the self-seeding - crape myrtles springing up everywhere! - that I dislike the most (and it's the bright bloom at this time of year that my husband likes). I am beginning to see them the same way I see Bradford pears.

I would rejoice if not one more Natchez crape myrtle was planted. It can be a fabulous plant with deft formative pruning and the right location, but it rarely gets those two vital requirements.  

There's one weed in particular that is vexing -- it's very lacey, fairly dark in color and almost camouflaged against the dirt. Wide spreading. Any idea what it might be?

That be your prostrate spurge, m'hearty.

It's in a very narrow side yard. Thinking of replacing with a sweetbay magnolia. Would bald cypress or black gum work in only about a 6 foot wide space?

No. The sweetbay magnolia would be a much better choice. 

I like to grow rattlesnake pole beans each year in my raised bed garden with chickenwire around it. I never had a problem before, but this year, after they had grown so tall and beautiful and were producing great beans, something came along and bit through the stem of each plant about 2 or 3 inches above the dirt line. I don't need to tell you how heartbreaking it is to go out one day and find all the plants drooping on the trellis. Any idea what did this and how to prevent it next year?

Sounds like a rabbit to me. It might not be too late to plant again. (Though those bunnies will be getting hungry).

I am planning to plant some lavender around my mailbox - to your point above about when to plant, is it better for me to plant this next spring, or should I be planting this fall?

If you can find a lavender now, late summer is a good time. The key to lavender establishment in spring is to be gentle with the roots when planting, planting in a lime-amended free draining soil, a mulch of gravel or grit, and careful but not excessive watering at the rootzone, not the foliage. Irrigation systems will probably kill them. 

I had a row of thriving hydrangeas until my neighbors took their trees down. Now they get way too much sun. Could that be why their leaves are covered with brown spots? Should I just move them?

Yes, I would move them in about a month. (You could start root pruning with a sharp shovel now). Place them where they will get afternoon shade. 

I have a container garden in the middle of the city. My three Knockout roses, which are in 24" pots all have Rose Rosette Disease, as do two of my four climbing roses, which grow on a trellis. They will all be removed before the first frost. I will replace the climbing roses with climbing hydrangeas, but am struggling with how to replace the Knockouts. I was thinking of some kind of evergreen tree, like Italian Cypress or some sort of pine tree. I could use something tall to make up for the lack of ornamentation. What do you suggest?

A chamaecyparis might work, as would an upright, blight resistant cultivar of boxwood. Italian cypress won't work in the Mid-Atlantic. And on that note, see you here next time, arrivederci. 

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Adrian Higgins
Adrian Higgins is The Washington Post's gardening columnist. Read his latest columns and stories here and follow him on Twitter.
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