Prevent unnecessary death.
Promptly plant nursery stock purchased earlier this spring. Wait until direct sun has left the site for the day; if torrid conditions are forecast, rig artificial shade for a couple of days (a beach umbrella, a sheet thrown over poles). Be generous with water for the rest of summer.
Plant tender summer bulbs (caladium, dahlias, etc.). For strong dahlias, dig holes a foot deep; set tubers in and cover with soil only partly. As the shoots grow, gradually add soil to the hole, always leaving the tops exposed; when the shoots are three inches above ground level, the holes can be completely filled. This presumes excellently drained soil, or the dahlias will drown.
Continue sowing a few green beans at a time - to allow continuous harvests later.
Last chance to plant these vegetables: limas, cucumbers, eggplant, squash, and pumpkins.
Sow chervil and cilantro repeatedly all summer to compensate for their tendency to bolt fast (produce flowers).
Sow more quick-growing annuals like cosmos and nasturtium.
Put cages or stakes around tomatoes, even if the plants are dinky. You'll be glad later.
Stake lilies. The stake should be as tall as the lily stalk will eventually become; tie stem loosely to stake as close to the top as possible. Staking a lily only to the midpoint is an invitation to a broken stalk. Old shoelaces, cut to appropriate lengths, are excellent ties - unlikely to cut into stems, as string often does.
Prune early-blooming clematis, such as C. montana, if desired to reduce the size. By pruning now, the subsequent new growth will have time to mature before winter and bloom next spring.
Prune azaleas and rhododendrons (except late bloomers), mountain laurel, pieris (andromeda). If you have vast plants, you can be radical. There is some risk in cutting an old plant to stubs, but most will sprout - the roots desperately want some leaves doing the photosynthesis thing. As a compromise, cut out a third of the plant. Beware: They may look like Addams Family oddities for a couple of years. As with early clematis, do this now or not at all.
Extend the azalea-rhododendron blooming time into July by adding these native American species and their hybrids to your garden: Rhododendron viscosum, arborescens, prunifolium.. The first two are particularly fragrant.
Mulch, mulch, mulch. The main reason is to help the soil retain moisture and remain cooler. Weed suppression (not elimination) is a bonus. If using wood chips, a pile that has been sitting around since last year is preferred to fresh chips. The latter are likely to deplete the upper layer of soil of nitrogen, which wood needs to decompose; comprehensive garden centers sell pellet-form nitrogen, which can be used to compensate - but don't overdo it or you could have luxuriant foliage and minimal flowers.
Know what a flock of starlings pecking at your lawn means. They most likely are there because the lawn is infested with grubs, the larvae of Japanese beetles and other creeps that eat the roots of grass. The chemical halofenozide (marketed as Mach II, an ingredient in lawn insecticides) is effective at this time of year; it has been ranked on the relatively mild side for chemical ill effects, but should not be used near bodies of water. Biological controls include BioNeem (effective in late spring), beneficial nematodes (mid to late summer) or milky spore (which can take a year to be effective, and then only against Japanese beetle grubs).
Deadhead columbine that have finished blooming to prevent seedling takeover next year. (Or leave one or two pods on the most attractive plant to ripen and self-sow at a reasonable rate.)
If chrysanthemums and asters have grown enough since the last pinching, do it again. There should be at least two, preferably three, nodes (junctures on the stem from which new side shoots will emerge) remaining after pinching.
Keep the lawnmower on a high setting. Cutting long grass too short runs the risk of "sunburn."
- Michael Martin Mills