Ask the Gardener: Protecting trees while putting in a new driveway

Ask the Expert Gardening
Jerry Caron's granddaughter, Brooke, with the Heavenly Blue morning glories in his Berkley yard. JERRY CARON

What to do this week Keep harvesting vegetables such as potatoes, rutabagas, radishes, and tomatoes, if you still have them. This is the best time of year for making new garden beds and many other outdoor projects. Combine dividing perennials with planting spring bulbs in the same holes, as they bloom at different times. We spurn the noisy and polluting leaf blower and instead lower our mower blades to an inch and a half and mow leaves directly into our lawn so they fertilize the grass roots naturally. Where leaves are deep, we collect them into 3-foot piles and leave them to decompose for 15 months into 1-foot piles of sweet-smelling, weed-free compost, which is the best soil amendment and mulch that I know. To speed things up, we mow the leaves first and add a shovel of soil rich in organisms. We add minimal fuel to the mower so we can idle it after its last run until the gas tank is empty when it’s time for winter storage.

Q. How close to a large tree can I put in a new driveway without killing it? Will a gravel driveway do less harm than macadam?

L.P., Milton

A. The drip zone, the outer edges of a tree’s leaves, has been adopted by the International Society of Arboriculture as the guide for protecting trees during construction. Temporary fencing should be installed around the drip zone to prevent heavy equipment from driving over roots and crushing them while at work. Another formula for measuring the tree protection zone, or TPZ, is 1-foot distance for each inch diameter of its trunk, according to certified arborist Todd Caswell of Natural Tree & Lawn Care of Avon. George Barth of Hartney Greymont Tree Service of Needham and Concord suggests using an air spade — a tube that blows soil away from roots — so they can be pruned by an arborist, causing much less damage than having tree roots torn off by an excavator. Barth said even gravel driveways can be deadly, typically involving 10 inches of excavation and a layer of sand packed down by heavy equipment beneath the gravel.

Most tree feeder roots live in the top 16 inches of soil. “The closer the driveway comes to trunks, the greater the chance of killing the trees,’’ said John DelRosso, chief arborist of the Arnold Arboretum, adding that damaged trees might take a couple of years to die or blow over.

Q. By last September I had hardly any blooms on my Heavenly Blue morning-glories, so in frustration I cut the plants off at the base and within a few days they were blooming like crazy! I couldn’t tell you why — maybe shock. This year I decided to go with a different variety: a Harris variety called Ipomoea Grandpa Ott. They started blooming in June!

J.C., Berkley

A. Thanks! I heard from many morning-glory fans. Cutting back reluctant bloomers of all kinds is an old trick that sometimes works. Heavenly Blue is one of the slowest blooming varieties but the best blue in color. I am afraid our season is often too short for it, so do keep experimenting with faster blooming varieties.

Q. Is it too late to propagate English ivy from cuttings? Mine, a groundcover, has had a growth spurt and I would love to cut a lot of it and use it in potting soil in containers to get a jump start for the spring. However, I think I have run out of time.

D.B., Westwood

A. Maybe, but do you really want to? When questing ivy finds a vertical object, it climbs. Universities have spent fortunes to have them removed from their hallowed walls because they can damage architectural brickwork. In fact, I’ve concluded that most non-native vines are bad news, be they ivy, trumpet vine, porcelain berry, Asian honeysuckle, bittersweet, black swallowwort, or even fall-blooming clematis. They are the sea serpents of the plant world. Stick them here, and they snake along underground to pop up in your lawn over there. Pull them out, and they resurface. This is the time of year when I clip stems near the base of invasive vines and immediately stick the fresh-cut end that is still attached to the ground into a bottle of glyphosate for a second to poison the roots without harming the unintended victims of spraying.

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