Keys to Growing Healthy Trees

Jun 22, 2016 | Rebecca in the Mercury News

Knowing soil types and water requirements may help us grow healthy vegetable gardens and flowers, but it is also vital when it comes to trees.

Igor Lacan, environmental horticulture adviser for UC Cooperative Extension, says as we move toward warmer temperatures with less predictable annual rainfall, we will need to make smart choices about our landscapes.

“Even in a drought, it is essential to prioritize your trees,” Lacan says. “Trees not only support our native birds, bees and wildlife, they provide major ecosystem services to us as well. Urban trees lower the ambient temperature, thereby reducing the need for air conditioning, sequester carbon from the atmosphere, provide stormwater capture, decrease pollution and enhance the property value and aesthetics of your home.”


In order to practice responsible irrigation — using enough water to keep a plant alive and no more — knowing your soil type really does matter.

Soil type, or texture, refers to the proportions of sand, silt and clay particles in its makeup. Sandy soils are coarse and drain quickly. Plants in sandy soil need frequent watering and may need fertilizer.

Clay particles are very fine and become glued together when wet, and although clay soil can be slow to drain, it retains moisture and minerals, requiring little to no fertilizers.

The roots of newly planted plants may have a harder time getting started if the soil is hard and dense, but once established, plants tend to thrive in clay soils.

Silty soil is found along our riverbeds, lakes and other riparian areas. Particles are smaller than sand but not as fine as clay. It drains well and has good nutrient retention.

Loam represents a combination of sand, silt and clay and most of the Bay Area has clay or loam soil.

To tell what kind of soil you have, moisten a handful of it and give it a firm squeeze. If it holds its shape but crumbles when you give it a poke, you have loam. If it holds its shape without crumbling, you have clay. If it falls apart as soon as you open your hand, you have sandy soil.

Knowing your soil type will guide you in how much water to apply and how often.

To gauge soil moisture levels, you will need to dig down to the root level. For trees, use a shovel or an auger to get 12-18 inches below the surface. After watering to this depth, soil should be moist but not drenched.

For mature trees, deep water infrequently, about once a month. Imagine refilling a 12- to 18-inch deep water reservoir around the tree’s roots.

It’s important to water beneath the entire canopy. Installing a Tree Ring Irrigation Contraption (TRIC) is a great way to accomplish this.

Newly planted trees may need only 10-15 gallons per week, but they may need additional water in extremely hot weather.

In all cases, a good rule of thumb is to water deeply and observe your tree. If the tips of the leaves and branches start to droop, it’s time to water again.

You will then be able to properly set up your automated irrigations systems. But remember, they need to be changed seasonally as the weather and temperature fluctuate. Online watering calculators can also be helpful.