Water needs vary dramatically from season to season. In early spring typically we get enough natural rainfall for established plants to go it on their own. Newly planted additions will probably need a good soaking once a week or so to help their roots get established.
As we get into the summer, plants use much more water. The size and number of leaves has increased, the temperature has increased, and the evaporation has increased. We usually need to supplement natural rainfall at this point, even for established plants. During these times plants are trying to build their “reserves” for next growing season.Most plants need about one inch of rain per week. On top of this is the fact that soil can only absorb about one quarter inch of rain an hour (this will vary somewhat with soil type). So no matter how hard it rains in an hour, the plants are only able to “grab” about one quarter inch of it. It is rare for us to get a four hour steady rain once a week in the summer. We need to help our plants by making up for any shortfall. I often tell people to pretend it doesn’t rain at all in the summer. Rarely will you be able to over-water a plant in the landscape. Once every five to seven days give plants a very, very thorough soaking. An inch of water applied over a four hour period with a sprinkler, or better yet a soaker hose, will keep your plants happy and healthy over the hot months of the year.
Containers are a horse of a different color. Think about it. Plants in containers are completely dependent on you to provide the moisture. No reserve in the surrounding soils to draw from. The rule of thumb is to water thoroughly every sunny day during late spring and summer. In early spring and into the autumn you may not need to water quite as frequently. In the early spring most plants are not yet large, meaning they do not need as much water yet. This, coupled with the cooler days, means that you may only need to water once every two to four days. Let the plants, not the calendar, be your guide.
Lots and Lots! For a hanging basket one gallon is appropriate. On larger containers two or three gallons may be needed. All too often we see people watering with a cute little watering can or a teacup. NOT ENOUGH! You need to water until excess water comes out the bottom of the container. And don’t be fooled by water sneaking down between the pot and the soil mix. If water starts coming out the bottom almost as soon as you pour it in the top then the soil has “shrunk” back away from the edge of the pot. If this happens set the pot in a saucer or inverted trash can lid filled with water. This will re-expand the soil to where it should be.