Today I learned how to effectively water a tree.
Although many trees have tap roots, most of an established tree's roots are in the top 6 to 12 inches of soil, and extend out horizontally from the trunk, far beyond its canopy or drip line (the area under its branches). April Rose, Executive Director of TreeFolks, likens the relationship of a tree to its roots as "a wine glass sitting on top of a dinner plate." And despite its name, the root flare on a tree is primarily trunk tissue, not root tissue. Trunks don't take up water; roots do. So watering a tree directly on top of its root flare is a lot less effective than thoroughly watering the soil underneath its full canopy. In fact, keeping the trunk's root flare constantly moist can cause fungal rot.
To keep trees well hydrated in the home landscape during this exceptional drought, water according to age of the tree and the diameter of its trunk in inches. There's a nifty chart on this page from the City of Austin. Baby trees need about 10 gallons per diameter trunk inch every two to four weeks for the first two years. Established trees need fewer gallons per diameter trunk inch (although more gallons overall) and tolerate less frequent watering. Both young and old trees benefit from slow watering methods which allow water to penetrate about 8 inches into the soil under the canopy around the perimeter of the tree. You can use a screwdriver or soil probe to gauge how deeply the water is penetrating the soil; the screwdriver will penetrate easily in moist soil. Mulching under the tree's canopy to a depth of three inches helps hold moisture in the soil and reduces weed growth, but pull the mulch back away from the trunk about 4 inches, to keep the trunk and root flare nice and dry.
For more information on growing healthy trees in Central Texas, visit the TreeFolks resource page.
(Today I Learned features a nugget of information I learned during training to become a Travis County Master Gardener.)
Words and photos © 2009-2011 Caroline Homer for "The Shovel-Ready Garden". Unauthorized reproduction is prohibited.