What to Do in the Garden in May

Here are some garden tips for May for the metropolitan Washington region from the experts at Behnke Nurseries.

If you've got another one to suggest or even a point you'd like to disagree with, let us know in a comment. Behnkes is working to compile the very best advice for every month of the year, and we welcome your input!

Perennials and Flower Borders

• Continue planting perennials and moving your extras to better spots in the garden—just be sure to water them every day for the first week (if it doesn’t rain) and water them weekly for the rest of the season (again, if it doesn’t rain).
• Deadhead spring bulbs as they fade, but don’t remove the foliage—the bulb needs to withdraw energy from the leaves to make new bulbs for next year.
• Give borders a nice new edge (if you didn’t do it last month) for a clean, neat look and easier weeding. (Here’s how to edge naturally.)
• Apply 1-2 inches of mulch (if you haven’t already), and we recommend shredded hardwood.
• There’s still time to divide perennials IF they don’t bloom in spring and IF they’re still relatively short (no taller than 5 inches).
• To prevent flopping of taller perennials, especially the ones receiving too much shade, cut them back by half in May. Prime candidates for this are Asters, tall Sedums, Monarda, Garden Phlox, and Purple Coneflower.
• Start pinching your mums (and other tall perennials that bloom in late summer) to produce fuller plants and prevent flopping.
• Stay on top of the weeding!

Annuals

• Time to buy and plant summer annuals, and start watering and feeding.
• Time to plant tender bulbs like cannas, elephant ears, dahlias and caladiums.

Trees and Shrubs

• After Azalea blossoms fade, it’s time to invigorate the full-size ones with renewal pruning. That means removing one-third of the stems completely—to the ground, or close to it, choosing the oldest for removal. If you need to reduce the overall size of the shrubs, the remaining, more vigorous stems can also be shortened. Small stems (less than pencil size) should be removed. Other shrubs that benefit from renewal pruning include Abelia, Deutzia, Forsythia, Mockorange, Spirea, and Weigela. With all of these, wait until the blooms have faded.
• For maximum blooms next year, remove the faded lilac blooms.
• Prune away dead, damaged and diseased stems and branches when you notice them.
• Feed your roses once this month (then again in June and July and once in October).
• Plant new shrubs and trees, watering well and continuing to water weekly through their first season in your garden, unless it rains (and thunderstorms don’t count!).

Edibles

• In early May, do your garden prep if you haven’t already.  For most gardens that means adding compost (Leafgro is great), turning it in gently rather than tilling it.
• Mid-May is the traditional time in our region to plant summer vegetable seedlings . Click here to see a list of heirloom and unusual varieties of tomatoes and peppers offered by Behnkes.
• Plant woody herbs—like rosemary and thyme—in early May.
• Seeds and tiny seedlings of fleshy, nonwoody herbs like cilantro and basil are good eating for birds, so keep them protected from birds until they’re larger.
• If you haven’t already planted your potatoes, do it now.
• Time to direct-sow beans in the ground.
• Beans need really warm soil, so wait until late May or early June to plant them, depending on the kind of weather we’ve been having.
• Weed, weed, weed—they compete too darn well with your desirable edibles. You can just snip off the annual weeds, but perennial weeds really need to be dug up.

Lawn

• It’s a big month for mowing—and the good news is if you leave grass clippings on the lawn, they’ll feed your turf (literally, about a third of its yearly requirement of nitrogen!)