Answer: The combination of heat and increased plant size can put a lot of stress on the plants. This is especially true for plants in containers where the root zone may be limited and for plants that have not rooted into the ground as well as they should have. The best method of watering plants in the ground is to water deeply about once a week, and possibly a bit more often during very hot weather, but avoid that daily light sprinkling. The idea is to moisten the soil deep enough so that the roots grow deeper to where the soil stays moister longer. This avoids that daily wet/dry stress. (You can use your finger or a shovel and dig down to see how effective your watering is. The rule of thumb is an inch of water a week, possibly more in hot weather.) Plants should also be mulched with an organic mulch to help keep the soil cool and moist. The planting area should also be well prepared by adding copious amounts of organic matter (such as compost, rotted leaves, or aged stable manure and bedding) to help improve the soil's capacity for holding both air and water.
By midsummer, plants in containers may normally require daily watering if they have filled in their pots or if they are in a very hot and/or windy location. Increasing the pot size and providing shade for the hottest part of the day can help with this. Be sure too when you water that the water is soaking into the soil and not running out between the edge of the pot and a dried out soil ball. If this is happening, you can try setting the pot in a pan of water to soak or wick in moisture and remoisten it thoroughly; when moist, take it out and allow it to drain so it is not waterlogged. Then water carefully thereafter. Some gardeners also report success using the water holding polymers which are mixed into the potting soil so you might experiment with those next year.
Finally, plants show the stress worst in the late afternoon. Often you will find that they perk up again as soon as it cools off, and look better in the morning than in the evening. If this is the case, you may actually be watering enough.
Having said all that, I should mention that pansies really prefer cool weather and will sulk when the temperatures turn hot. Some gardeners use them as spring and fall plants and replace them for the summer for this reason. You might try cutting them back in late August and see if they rejuvenate to make a nice fall show; if they are in pots you might try setting them in a cooler location until the temperatures begin to come down.
Snapdragons also tend to bloom well in the spring and early summer then take a bit of a rest and come back strong in the fall. Deadhead them (cut off spent flowers) regularly to encourage blooms. If it seems they are resting, cut them back to about four inches, topdress with compost and continue watering. They should come back to bloom for you in the fall as well.
Impatiens are tropical plants and enjoy the heat, so they are often a mainstay of the summer planting. They do best in shade to part shade, so if they are in sun you may find that they wilt no matter what you do until the weather cools off a bit.
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