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Ask the Gardener: How to prune a rhododendron

Plus, what to do in your yard this month and tips for transplanting trees and moving your houseplants outdoors for the summer.

Lane Turner/Globe Staff
A rhododendron in bloom at Arnold Arboretum.

What to do this month June is another busy, but satisfying month. Finish spring-planting projects, postponing longer-term ones until September, if you can. Sow warm-weather vegetable seeds such as corn, squash, and beans. Plant tomato, melon, squash, cucumber, pepper, and eggplant transplants. Plant tomatoes labeled as “indeterminate” in tomato cages. Tomatoes labeled “determinate” usually do not need staking. Buy and plant flowering annuals while the selection is good. Plant tropical bulbs such as dahlias and cannas. Stake taller, weaker perennials like delphiniums and peonies, or cut them for “bouquets of the broken” if you forget. Cut flowers with clashing colors, too, until your garden looks the way you want, and make flower arrangements from the discards. Diligently weed invasive species before they go to seed. Pull out spent one-time tulip bulbs by the stalks, but leave the foliage of more enduring daffodils to feed rebloom next year. Buy or plant container gardens. Take houseplants outdoors to a shady spot for a few days before moving them later into more sun. Water potted plants daily in hot weather.


Q. I have 12-foot rhododendrons. Last year’s drought caused a lot of dead branches. Should I cut them back now?

J.C., Newburyport

A. Yes. My garden has never had so much dieback. The tops of most of my roses are dead, but new canes are sprouting from the base. Many readers wrote that their forsythias failed to bloom, though they did leaf out. In addition to the drought, I think the biggest problem was the sudden one-night temperature drop to below zero that burst so many houses’ pipes in February during an otherwise mild winter. When you prune, cut leafless branches all the way back to a fork flush with another branch that has leaves. Don’t create stubs. Roses will often sprout new canes at the base, but rhododendrons won’t do this. Minimize fertilizing, but mulch and keep the newly pruned plants regularly watered.

Q. A friend let me dig up five Eastern white pines and four hemlocks from his woods for transplanting to my yard. They range from 5 to 10 feet tall. Assuming there’s no rain, how often should I water them? If they die, what types of trees would you replace them with that don’t need watering?


D.L., Milton

A. It is better to transplant trees in September, or else you will have to water them deeply and weekly through their first June, July, and August, when it almost certainly will not rain enough. In fact, new trees need daily watering their first entire month. Native trees that don’t need a lot of water after their second year include thornless honeylocusts, Northern catalpas, Northern red oaks, black oaks, hackberrys, Kentucky coffee trees, and hornbeams. Buy an inexpensive, circular plastic tree-watering bag and fill it weekly from a hose for the first 90 days, then discard it when it wears out. The bags save time and money because the water seeps out slowly toward the roots instead of running off. Incidentally, digging up a tree is no joke, because the roots can easily weigh 200 pounds, attached soil included. It’s usually a two-person job, so I dig trees less than 5 feet tall for transplanting because the roots are lighter. If they survive, they will make up time by growing faster than larger specimens.

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