Ask the Arboretum: Autumn gardening tips

As chief horticulturalist at Penn’s Morris Arboretum in Chestnut Hill, Vince Marrocco is responsible for keeping all 92 acres of the garden—from the rose garden to the oak allee to the herb garden—looking great all year long.

He’s been doing just that for the past 12 years, with great success.
So we here at The Current figured there couldn’t be a better person than Marrocco to answer some of our most pressing garden questions. In an occasional column that starts today, we’ll chat with Arboretum experts about garden design, plant care and yard maintenance tips—just about anything and everything that has to do with the plant world (we encourage you to send in your questions, too, so feel free to drop us a note at Fall is closing in fast, so in our first chat with Marrocco, our questions turned, naturally, to autumn—and what we should be doing now to make sure our gardens and yards look great when spring comes again.

Q. What are some good plant choices for people who want to get some autumn color in the yard—instead of the same old mums?
A. One thing they can do is to make sure they buy plants that have great fall color. People tend to buy plants in the spring at the garden center, and so most yards bloom in May and June and then don’t do a thing the rest of the summer. You can even buy plants that may not bloom in the fall, but have a great fruit set or just good fall color. Native viburnums have great fall color. There’s the Winterthur viburnum, which comes from Winterthur Gardens [in Delaware], which has a beautiful stop-sign red color. That’s a great plant. It also has beautiful, purple, glossy fruit. There are also native deciduous hollies that you can grow for winter interest. They lose their leaves, so in the winter you can see this very heavy berry set. Birds mostly leave them alone because they’re pretty distasteful, at least until January. So it does provide forage for the birds in the winter.

Q. What should gardeners be doing to their beds this time of year?
A. One of the best things you can do is put down mulch. A lot of winter weeds are germinating right now, but they need sunlight to germinate. If you put down a layer of mulch, the seeds can’t germinate, and you’ll have fewer weeds in spring and summer. That’s what we do. We’ll also be cutting down and cutting back all of the perennials that are done for the year. One of the things that can happen, if you leave your larger perennials there, they’ll end up providing a nice home for mice, who will then eat your plants until there’s nothing left of them. If you don’t cut them, mice love it. All that foliage, laying flat, that gives them places to go.

Q. What about lawns?
A. In terms of lawn care, this is the perfect time of year for fertilizing and aerating and overseeding. That’s one of the big things we do right now. We’re aerating, where we have a machine that pokes holes in the soil and provides higher oxygen content in the root zone. Although plants give off oxygen, their roots require oxygen. If you have a higher oxygen content, you’ll get a much thicker root mass, and a much healthier lawn. You should fertilize two times in the fall. The first time is around Labor Day, when you should put down a high-nitrogen fertilizer. That helps the lawn green up after a pretty harsh summer. August here was pretty dry, and a lot of lawns around here turned brown and yellow. Around Thanksgiving, then, you should put down a winterizing fertilizer, where nitrogen is the lowest [of the fertilizer content] and the phosphorous and potassium levels are high. That will encourage root growth and hardiness, and if we have a severe winter, the lawn is less likely to have a bad winter.

Q. Is now a good time to prune back shrubs and trees? How about dividing perennials?
A. That’s a tough call, and it kind of depends on the species. Usually I wouldn’t prune now, because you leave an open wound when the plants are transferring a lot of stored energy reserves into the trunk, and an open wound can bleed heavily. This is a time of year when there’s a lot of sap moving, and it might be better to wait until summer or the dead of winter. That being said, the best time to prune is when you’re actually there, ready to prune. Because if you’re not there, later on, when it might be a better time, it won’t get done. … Sometimes, if you have the time and you’re there, it’s just better to do it than not to do it at all. As for perennials, it can depend on the species. We divide our irises in the fall. But one of the problems with dividing in the fall is that if you do it too late, the roots won’t have grown enough to stabilize the plants in the soil, and you can actually get heaving of plants out of the ground through the freeze-and-thaw process. So unless you can do dividing early enough in the fall, you might not want to do it until spring.

Originally published on September 21, 2006.

Originally published on September 21, 2006