With temps in the 80's and 90's, plants, especially perennials, tend to dry out exceedingly quick. Make sure to monitor plants by sticking your finger in the soil next to the plant. You want the top 2 or 3 inches of the soil to be dry. If it is dry below that, you need to water.
New plantings need to be monitored more carefully for the first full year. Established plants, such as Hemlock, Oak, Redbud, and Hydrangea, all prefer extra moisture during dry periods. Water these plants once a week during hot and dry weather, especially during August, September and October. All evergreens really like additional water later in the season to keep them healthy during the winter months. In warmer weather, it is best to water in the morning hours to give plants time to soak up the water before the hot sun makes an appearance. Morning watering also helps to prevent the plants from wilting in the blazing afternoon heat. Be sure to check for any local watering restrictions in your area.
The proper technique for watering your plants is to take off the nozzle from the hose and visit each plant with the water running close to the base of the plant with about three quarters water pressure. A steady stream of water to the base will allow for a deep watering and enable the water to reach the root zone, not the just the leaves.