Mosquitoes breed in standing water, so empty plant saucers, pails, old tires -- anywhere water collects after a rainstorm. Change the water in pet dishes and bird baths every few days. Use mosquito dunks or add fish to ponds, or use an aerator because mosquitoes won't lay eggs in moving water.
Cut Back Shrubby Perennials
Once they're done blooming, cut back early-flowering perennials like catmint (nepeta), salvia, candytuft, cranesbill, and creeping phlox to tidy up plants and perhaps encourage more blooms.
Slugs can cause a remarkable amount of damage, so be on the lookout for their silvery trails. Then go out in the early morning to hand-pick. Or use traps to catch them. An inverted terra cotta pot is an attractive hiding place for them to get out of the hot sun; remove the slugs in the afternoon, before they crawl back out. Iron-phosphate-based baits are another good option.
Install Plant Supports
Tall, top-heavy flowers like delphiniums are prone to flopping when in full bloom. If you haven't done so already, install plant supports to keep them upright. It's best to install the supports in spring soon after the plants sprout, but there's still time to do so now.
Plant Wisely Around the House
Watch where rain splashes off roofs and from downspouts, and avoid planting anything there that will be damaged. Check foundation plants to be sure they are getting water when it rains; if eaves prevent rain from reaching them you'll have to water them by hand. Keep plants a few feet from the foundation, and choose plants that won't get too large for the site once they're mature.