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Urban Forestry
Arborist's Office
206-684-TREE (8733)
Street Tree Planting Procedures
New Tree Planting
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Trees Approved with Reservations
Watering Newly Planted Trees
Watering Established Trees
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Tree Pruning Guide
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Watering Established Trees

Healthy Red Maple - leaves are healthy green - canopy is full - leaves are uniform size

Stage 1 - leaves are lighter green and look wilted - canopy is beginning to thin - some leaves are smaller in size

 

Stage 2 - all symptoms in Stage 1 are present, but leaves are showing fall color in summer, as tree is shedding leaves to lessen its water needs

 

Stage 3 - tree has shed leaves and now branches are dying back - at this point a tree can suffer irreparable damage or can die.

July 28, 2009

Once again, we’re experiencing a Seattle summer with an occasional bit of cloud cover, but very little rain. The situation has also been compounded by record high temperatures over an extended period of time. Drought conditions can have substantial negative impacts on both newly planted and established trees, resulting in decline, pest problems and other damage from which the trees are sometimes unable to recover.

Trees over twelve inches in diameter are generally considered to be impossible to replace, and very large trees may take two or more human generations to regrow. Please consider watering your larger landscape trees if they begin showing signs of stress, such as falling leaves, thinning crown or early fall coloration.

If you don’t have an automatic sprinkler system, the best way to water is to use a soaker hose or other type of drip irrigation that you turn on and off manually. Sprinklers are much less efficient for applying water, as some water is lost to evaporation or application to impervious surfaces. Even a simple garden hose running at low volume, and moved frequently, can provide a good soaking.

What area should I water?

When watering an established tree, concentrate the water in the soil areas beneath the tree, that are shaded by the canopy. Don’t water out beyondthe “drip line” and try not to water within 2 or 3 feet of the trunk. Don’t spray foliage when watering, as water droplets can result in pest damage or sun damage to the leaves.

Remember that water generally does not move sideways in the soil. Water directly over the top of the critical areas of the root system for best results. If you are watering a newly planted tree, the most important area to keep moist is the root ball of the tree and the soil that was excavated when it was planted. Focus on keeping the tree alive until fall, when the tree can begin to expand its root system into areas that are moistened by fall and winter rains. Using a drip irrigation bag (such as a TreeGator or OozeTube) or even a 5 gallon plastic pail with a small hole drilled in the bottom can be helpful in applying an appropriate amount of water and minimizing water waste because of runoff.

When should I water?


Trees generally replenish their water deficits during the night time hours. If you water between 10:00 pm and 8:00 am, you will avoid the time of day when the evaporation rate is the highest. Watering in the evening allows more water to penetrate the soil and be used by the tree. Night
time watering also does not increase the time that foliage of understory plants is damp, as dew is generally already present.

The next best time to water is when foliage is dry and the potential for evaporation is beginning to decrease – this is generally in the late afternoon. Foliage should be given adequate time to dry, before dew begins to form in the evening, in order to minimize the potential of pests or diseases that require longer periods of moisture to become problematic.

How often should I water?

For a newly planted tree, 5 gallons of water for every inch of diameter, once a week, should be an adequate amount. Remember that for approximately every 20 degrees of temperature increase, the amount of water used by a tree will double. During extended hot spells, adjust watering frequency accordingly, as a newly planted tree can use all of the water that’s available in its root ball in as little as one day. Trees with root systems that are in confined spaces or trees that are surrounded by a lot of paving are particularly at risk from drought stress.

For older trees, an inch of water in the area under the dripline, once a week, should be adequate. Trees with limited rooting areas or on steep slopes should be monitored closely to ensure that an adequate amount of water is reaching the root system. Water more frequently in areas with very sandy soil.

What else can I do?

Wood chip mulch in the area under the dripline of a tree is one of the best water conservation methods available. Mulch reduces competition from weeds, keeps the soil cooler, and reduces evaporation. Mulch also amends the soil with organic matter as it breaks down, which improves soil structure and water infiltration.

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