Arizona Gardening: Watering guidelines depend on plant and temperature - East Valley Tribune: Get Out

Arizona Gardening: Watering guidelines depend on plant and temperature

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Posted: Friday, June 27, 2008 11:50 am | Updated: 9:25 pm, Fri Oct 7, 2011.

I hear a lot of different opinions about how often and how much to water plants during the summer. Are there some guidelines on summer irrigation?

Q: I hear a lot of different opinions about how often and how much to water plants during the summer. Are there some guidelines on summer irrigation?

A: There are semester-long college courses that teach those principles, and you’re asking me to give advice in one short column. However, here are a few guidelines.

First, we have to know the needs of the plants. If they are heavy water users like ash trees and philodendrons, medium water users like sissoo trees and pittosporum, or arid trees like ironwood and paloverde — that will tell us how often to water.

How is the water applied? If water is applied by 6-inch to 8-inch-deep flood irrigation every two weeks, then that is much different than if we are applying water by a drip system that applies 1 gallon to 4 gallons per hour. Obviously then, 2 to 3 drip emitters putting out 1 gallons to 4 gallons per hour will need to run all day compared with a few hours for 6 to 8 drip emitters putting out 10 gallons per hour. When, or how often, plants are irrigated should be determined by temperature, not by how the water is applied. Example: 6-inch-deep flood irrigation will wet the entire root ball down past the root zone, whereas the drip irrigation usually wets only the root zone a few inches and deposits the salts in the water among the roots.

Key: The length of irrigation should remain the same, only the time interval between irrigations will change according to the seasonal temperature. During the 50 degree to 60 degree days of December and January, plants may only need watering once a month for trees, once every two weeks for shrubs and once a week for flowers and vegetables. During 110-degree days during June and July, irrigations may need to be weekly for trees, every three to four days for shrubs and daily for flowers and vegetables.

Where the water is applied makes a huge difference. Certainly when a tree or shrub is initially transplanted the drip emitters or bubblers are focused near the edge of the root ball because that is where the roots are. Every four to six months the irrigation system should be modified to apply the water out from the trunks 12 inches to 18 inches, and the roots will seek out and follow the water. More emitters should be added as the root zone circle enlarges. Shrubs should also be moved back, but not as much as trees. The root zone of flowers and vegetables will not increase much and will be removed within six months anyway, so they do not have to be moved outward.

Knowing the needs of the plant types is also important. Tomatoes, veggies and most seasonal flowers need to be watered daily during the summer. Citrus trees and hibiscus do not like wet feet, so allow them to dry out somewhat between irrigations.

Guideline: Use a T-handled probe or long-handled screwdriver. If the soil is dry down 1 inch for veggies and flowers, water. If the soil is dry down 6 inches for shrubs, water. If the soil is dry down 1 foot under trees, water.

Contact writer: john@johnchapman.com

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