December 25, 2012 by

Omaha’s thousands of trees are in danger. This summer’s unprecedented heat and drought have put even mature, established trees in peril. Trees cannot endure a period of extended drought without help. If trees are not rehydrated soon, they will not be able to survive the winter, let alone fight off insects and disease next year. If the Ash Borer reaches Omaha, our ash trees may be unable to take up the nutrients they need to fight the insects.

As early as August, many trees in Omaha began “shutting down” in an attempt to conserve water. When this happens, growth ceases and plants prematurely lose their leaves. Trees already have next year’s buds. The drought not only affected last year’s tree quality, but has the potential to significantly affect trees’ appearance this year. This drought is unprecedented to our generation. Most trees have not experienced summer conditions like this before.

There’s still time to save Omaha’s trees. It’s important that tree owners take the initiative to water trees. It’s a misconception that large trees have roots deep enough to get to underground water. The majority of feeder roots are actually in the top 12-16” of soil. Established trees (5 years and older) are best watered with soaker- or drip-irrigation hoses. A regular hose running at a trickle is much less effective, as the water often runs beyond the target zone and pools in unhelpful areas.

Water should not be targeted against the trunk of the tree. Tree trunks can become subject to disease and insect problems if moisture is concentrated next to the trunk. A tree’s “root zone” actually spreads 2 to 3 times wider than the tree’s canopy. Water must be applied directly to this target area. Watering for short bursts can lead to additional drought damage. Shallow watering forces oxygen out of the soil and results in oxygen starvation of a tree’s roots. What our trees need now is consistent, deep watering.

Trees need 10 gallons of water for every inch of diameter of trunk, measured 2 feet up the trunk of the tree. For example, a 7-inch diameter tree requires 70 gallons of water in each section of the root system covered by the soaker hose or stationary sprinkler. A hose open at half pressures takes five minutes to produce 10 gallons of water. Therefore, each section of a 7-inch diameter tree’s root zone must be watered for 35 minutes. It may take several days of watering to cover the entire root zone.

If there’s no rainfall, trees should be watered once a week during the growing season, continuing on a regular basis until rain returns. The arrival of winter does not mean it’s time to stop watering. Winter drought can affect both evergreen and young hardwood trees. Water once or twice per month between October and March on warm days as long as the ground is not frozen.

The best time to water is at night between 9pm and 8am, as trees refill water reserves during the night hours. Watering at night allows for effective use of water with less loss from evaporation. This assures that more water moves into the soil and tree. The second-best time to water is late afternoon.

The best way of knowing if you’re getting water deep enough into the soil is to use a screwdriver. It should glide through the soil in the area you just watered to the depth of at least 8-10 inches with little or no resistance. Don’t be surprised if you hit concrete-like soil about 3-4 inches down. Water in shortened sessions until able to get water to penetrate. Don’t be surprised to find the soil almost repelling the water until sufficiently hydrated.

Younger trees require a similar approach for watering, but the root area is much less. A tree needs about one year per inch of trunk at planting time to establish a root system. It’s very easy to overwater the soil surrounding a newly installed tree. The root mass and a few inches beyond are critical areas during the tree’s establishment period. Every year, roots expand into the surrounding soil, creating a wider area to be hydrated in dry periods.

Here at Lanoha Nurseries, we are happy to answer questions and walk tree owners through the watering process. We care about all trees and want to ensure that Omaha’s trees stay safe and healthy.

Call to speak with a Lanoha professional at 402-289-4103 or stop in the Garden Center at 192nd and West Center Road.