It’s always difficult to tell people how often to water because it depends on if the plants are mulched, as well as on heat, rain, and how much you give each plant when you do water. Naturally, a good rain will help reduce the need to water. Keep in mind that a quarter of an inch of rain won’t get down very far into the soil. Mulching around the plants will help hold water in the soil, and we definitely recommend it. But the general idea is to water deeply when you water rather than watering often. You want the roots to follow the water down into the ground rather than staying up near the surface. Newly installed plants should be watered very well when first planted.
Check Moisture Daily
We also recommend checking moisture daily- lack of water and overwatering are the leading causes of death for new plants. Just keep an eye on the plants, and they will generally start to wilt when they need watering. If you are not sure, stick your
fingers in the ground by the plant. If it feels moist, it probably does not need water. If it’s dry, water it. If your fingers find that the soil is really wet, but the plant is wilting anyway, you may be overwatering. A plant that has been too wet for too long will wilt and turn brown just like a plant that has dried out.
Soak Areas Well When Watering
In times of moderate to severe drought, like we experienced previously this summer, even established plants need supplemental watering. But do NOT water every day (except for annuals in containers or newly seeded grass). In dry conditions, nearly everything will do best with a good soaking every few days.
What is a good soaking? Any slow release of water that can flow deep into the soil to encourage root growth. This can be from: a tree bag (pictured below)- 20 gallon bag that slowly releases water over several hours, a hose on trickle- 1/4 to 1/8 flow for 15-30 minutes, or 
slowly watering with approximately 10-20 gal per tree, 5-10 gal per shrub, and 1-5 gal for perennials.
When watering new trees and shrubs, soak the area when planting to eliminate air pockets. Water again in a couple of days, then water twice a week for two weeks. After the first couple of weeks, water deeply once a week for the rest of the first growing season to encourage deep root growth. Newly planted perennials may need more frequent watering.
How Often to Water?
Depending on the variety, soil type, and sun exposure, newer plantings might require a deep soaking every 2-10 days. And don’t forget about your more established plants too! Especially during drought conditions, Dave suggests watering every 10-20 days for established trees, every 7-10 days for trees planted last year, and every 2-7 days for perennials and shrubs. WATER DEEPLY, NOT DAILY!