What kind of wisteria should you buy?

Asian wisteria is difficult to control but beautiful if it blooms.atiger/shutterstock

What to do this week Deadhead spent flowers for neatness and to encourage more production. Harvest vegetables promptly for the same reason. If vegetables get too old or too big to eat, harvest them anyway to encourage new growth. Check to see whether potted plants need water in hot weather by lifting them; dry pots will be lighter. Sow more carrots now to harvest when they are 1 to 2 inches in diameter. Finish pruning roses and other shrubs and cutting back leafless stalks from last winter before August, so tender new growth is not stimulated too close to the onset of cold weather.

Q. I have a wisteria vine that grows bigger and bigger but has given me only two flowers. How can I get it to bloom? Should I get rid of it?

MARTHA FRIEND, Somerville

A. You can try fertilizing with potash, but I would get rid of it if you’ve had it for five years or more. Wisterias propagated directly from rootstock are often inferior and bloom poorly or not at all. It is worth paying more to buy a wisteria plant that is blooming in the nursery in May. You know it will flower, and you can see what you are getting. I avoid all common Asian wisterias because they get too big and aggressive, crushing structures like hungry anacondas. I have had good luck with Wisteria frutescens “Amethyst Falls,” however. This is a selection of native American wisteria that is not invasive like its Asian cousins, and is smaller and easier to train. All wisterias require weekly watering their first two years and a sunny location sheltered from winter winds.

Q. Every spring my “David” phlox starts out looking great, but then its leaves die from the ground up, and, though there are a few flowers, the plant looks sad. Should I replace it with another phlox? The garden also has a mix of daylilies, roses, and coral bells. How much water per week for flowers would you suggest?

HELEN LITTLEFIELD, Lowell

A. When people ask me about what fertilizer or disease control to add, my first answer is: Water, water, water. This is what plants really need, especially in summer. Your phlox probably starts out great because there is more natural rainfall in the spring. When the week is hot (more than 80 degrees) and there’s no substantial rain, your garden needs an inch and a half of water. That happens to be the measurement of a cat food tin, so I put an (empty) one in my garden and run the sprinkler until it is full. (You can also use a rain gauge.) When temperatures are cooler, plants need only an inch of water a week. Come autumn, that is usually provided by rainfall. I try to water in the morning because there is less evaporation and the leaves have time to dry in the sun, making them less likely to develop fungus problems. I also have drip irrigation in some of my gardens, which saves money and effort. It’s worth the investment. “David” is a top disease- and mildew-proof phlox cultivar. Don’t replace it. Just water it. (If new plants develop that are not white, incidentally, pull them out. They are rogue seedlings that will crowd up the more desirable parent plant.) If you can’t water your garden, stick to drought-tolerant plants such as sedum, daylily, butterfly weed, yarrow, and coneflower.