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Ask the Gardener: Limited times you can water? What you should prioritize

Ask the Expert
Hydrangeas-Purple-Pink-Blooms
Be sure to water ornamental shrubs that are in bloom, such as hydrangeas. Adobe Stock

What to do this week: Lawns grow slower in the summer, so don’t let lawn care companies mow them unnecessarily just to keep their crews occupied. Instead, let grass grow taller to produce deeper, more drought-resistant roots. Let biennials like hollyhock and forget-me-not go to seed if you want more of these plants next year, but deadhead or shear back annuals that have bloomed once already and harvest vegetables to keep them both producing. Weekly watering is more important than monthly fertilizing.

Q. With hot weather and limited watering hours in my community, I’m wondering what plants need the most water? And the least?

N.C., Ipswich

A. In my summer garden I prioritize annual flowers and vegetables, dahlias, garden phlox, astilbe, monarda, and anything growing in containers in sunny locations. I especially water crops while they are producing vegetables, fruit and berries, as well as young seedlings and any landscape tree or shrub planted within the past two years, as these are all still developing roots. I try to cluster plants with high moisture needs nearer to water sources, while plants that tolerate drought, including most established shrubs, are planted around the lawn, which I allow to go dormant each summer. However, I do water ornamental shrubs currently in bloom, such as hydrangeas and roses, along with important trees. Sometimes I just leave a dripping hose under the canopy. I do the same for town street trees in front of my house, which are a valuable part of my landscape and don’t get watered by the municipality.

Plants that naturally grow in wetlands logically need extra water in drought, including spicebush, Japanese iris, winterberry, swamp milkweed, clethra, swamp azalea, and joe-pye weed, to name a few. On the other hand, most plants native to deserts and sunny meadows like it relatively dry, including veronica, butterfly weed, sunflowers, sedums, yucca, salvias, and baptisia. When in doubt, Google a plant to find out its moisture needs, preferably before you decide where to site it. Use a rain gauge, or even an empty tuna can, to measure natural rainfall.

Q. I have a new house on the Cape that needs a 5-foot-high privacy hedge. Any suggestions?

C.C., Falmouth

A. Japanese holly (Ilex crenata) is becoming popular as a substitute for boxwood hedges because it has a similar dark, glossy evergreen appearance but grows faster. It reaches 5 to 10 feet in height and width, with small leaves and no thorns, and produces black berries for birds in the winter. It may be tightly clipped into almost any shape — including topiary! Select one of the faster-growing and more cold-hardy cultivars such as Convexa, which are widely available at Cape Cod nurseries. Be sure to dig organic matter, especially compost, into the soil when planting and to cover the soil with 3 inches of bark mulch to help retain moisture and keep the soil acidic. Don’t plant Japanese holly in Northern New England or in hot, dry locations.

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