Replace cool-season annuals (such as pansies, nemesia, and diascia) when they fade as temperatures rise. Plant heat-loving annuals (such as angelonia, petunias, or salvia) to keep your garden or containers looking great through fall.
Though the foliage on your spring bulbs (such as tulips, daffodils, or hyacinths) may be fading, wait to remove it until it's gone fully yellow and pulls easily out of the ground.
If there's still some resistance when you try to pull it ground, your bulbs are probably still gathering energy for next year's display.
Now's still a good time to map out your bulb garden if you haven't done so. Referring back to your diagram this fall can make planting more spring bulbs much easier since you'll know exactly where they're planted.
Try to go through your garden at least once a week and snip the dead, faded flowers from your plant. This process, called deadheading, encourages many plants to continue blooming. It prevents your plants from going to seed (so you won't have a crop of seedlings to pull) and can keep your plants from catching disease.
If you need to water your vegetable garden, it's best to do it deeply, but infrequently. That encourages your plants to develop deeper root systems so they can get through hot, dry periods more easily. Also: Use a soaker hose to provide water to your plants -- you'll save money on your watering bill (because you don't lose as much water to evaporation) and help keep your plants from getting diseases.
Stop harvesting asparagus and rhubarb this month -- they need a healthy crop of foliage for the rest of the summer in order to gather energy and give you good harvests next year.
It should be safe to plant warm-weather vegetables (including tomatoes, peppers, eggplants, corn, squash, and pumpkins) after your average last frost date.
In the northern half of this region, you may be able to get another crop of quick-maturing cool-season vegetables (such as radish, lettuce, or spinach) if you sow the seeds early this month.
If you didn't fully harvest any of your cool-season crops and they bolted (sent up a flowering stem), pitch them in the compost bin. In most cases, their foliage becomes too bitter for you to truly enjoy.
Test Garden Tip: If you're in an area where the summers get hot early, try planting lettuce, spinach, and other greens in the shade. They'll stay a little cooler -- and that may keep them going a little longer.
While you can divide most of your favorite perennials any time from spring to fall, it's easiest on the plant if you split it in the cool, moist season. Dig out and divide clumps of perennials to share with friends or to fill in gaps or holes in your garden.
It's a great time this month to take cuttings of some of your favorite trees and shrubs, including spiraea, serviceberry, chokeberry, butterfly bush, hydrangea, dogwood, and magnolia.
Watch for weeds -- they're much easier to pull when they're small. And if you remove them before they produce seeds, you'll have a lot fewer weeds to pull next season.
Prune spring-flowering shrubs such as lilacs once they've finished flowering. Now is the best time to prune them without affecting next year's floral display.
Watch for insect pests such as aphids, cabbage loopers, Japanese beetles. If the populations are small, you may be able to keep them under control by hosing them off plants with a strong stream of water (for small insects) or hand-picking them from the plants (for larger bugs).
If infestations are severe, consider spraying with an organic insecticide or insecticidal soap -- or attract birds and beneficial insects to attack the pests the natural way.
Slugs love wet weather. Watch for chewed holes on your plant leaves -- if you see slugs or signs of their damage, use a slug bait in your garden. There are traditional and organic types available.
Also: Encourage critters such as snakes and toads as both eat slugs.
Fungal diseases also love bouts of cool, wet weather. Some plants are very susceptible to certain diseases; spray crabapples, roses, phlox, bee balm, lilacs, or other varieties that always seem to get powdery mildew or leaf spot diseases with a fungicide during these wet periods.
Continue mowing your lawn as it grows. Encourage a lush, healthy lawn by keeping it about 2-1/2 inches tall. Longer grass blades often mean a deeper root system, so your lawn holds up better during drought. Plus, taller grass does a better job of shading out low-growing weeds such as creeping Charlie, plantains, and dandelions.
If you have a cool-season lawn and didn't fertilize at the end of May, you can feed it in early June. But don't fertilize after the first or second week; it's best to let your grass slow down a little and get ready for summer.
May and June are good times to aerate cool-season lawns because the grass is actively growing. Rent an aerator if your ground seems very hard and the grass has a hard time growing. The looser your ground is, the better job your grass will do at crowding out weeds.