Thursday, June 12, 2014

Planting Drought Resistant Deciduous (Broadleaf) Trees

We see far too many trees damaged or dying because of drought.  As tree lovers, this pains us greatly. Not only will these trees need to be removed and another planted, but several more years will be required for the tree to become large enough to provide significant shade or wildlife value.

When planting a tree in an area where drought is possible (most of the continental U.S. has experienced drought  over the last 5 years), two things are important.  First, the tree being planted must be carefully chosen.  Trees that are not well suited to an area are unlikely to survive.  Secondly, the tree needs to be planted correctly.

First, we have listed several trees that are well suited to drought conditions.  Trees are also sensitive to minimum temperatures, so USDA zones (see map here) are also included   Below our tree recommendations we discuss how to plant trees for maximum drought tolerance.

Drought Resistant Trees

The Thornless Honeylocust (Gleditsia triacanthos) is an amazing tree.  This tree is drought tolerant, salt tolerant, pollution tolerant, and fast growing.  You may have seen this tree without knowing, as it produces long, curved, leathery seed pods (that can be slightly annoying).  This tree grows essentially in the entire continental U.S., and grows well in parking lots and along streets (even if salt is sometimes used there).  Honeylocust is of value to wildlife, but also lets grass grow beneath.  For commercial properties owners in dry areas, these trees will survive drought, significant salt as well as pollution.  As these trees have deep taproots, they will grow very well with the Waterboxx and also will be less likely to buckle surrounding asphalt and concrete. The tree grows from zones 3-9, can grow 30-70 feet tall, and 50 feet wide.

Thornless Honeylocust is an excellent all around tree.  It is drought tolerant, has deep roots which don't buckle sidewalks or even brick streets (as seen here), and is easily established with the Waterboxx. 

The seed pod of the Thornless Honeylocust, just before maturity.  These can help you recognize these common trees.  The pods can be eaten by cattle.
The Common Hackberry (Celtis occidentalis) is another ornamental tree that grows throughout most of the continental U.S.  The trees are stated to need a minimum of 14 inches of rain annually, but the deep roots established with the Waterboxx may lessen that somewhat. The Hackberry produces small berries which attract many bird species (be forewarned if parking cars beneath it), and are also eaten by many mammals.  The Hackberry can reach 60 feet in height and spread and is well suited to urban conditions as well.  It grows in zones 3-9.

The Japanese Zelkova (Zelkova serrata) is a a beautiful street tree with excellent fall foliage.  It is resistant to Dutch elm disease, has excellent timber, and is pollution tolerant.  Although not native to the United States, it grows well here in zones 5-8.  It is drought tolerant once established and reaches 80 feet in height and spread.   The roots do tend to be shallow but this will be somewhat corrected with the Waterboxx.  Of note, the bark naturally peels and reveals a beautiful orange coloration beneath.

A beautiful, full sized Japanese zelkova tree, growing in the Orto Botanico in Florence, Italy.  

The Honey Mesquite (Prosopis glandulosa) grows well in the far south of the United States, and is best known for producing firewood using in barbecue.  It is extremely drought tolerant, but does have thorns so it is not a great sidewalk tree.  These trees grow about 35 feet high, and are very difficult to establish without watering.  Luckily, the Waterboxx will take care of that.

A long lived broad leaf option is the bur oak (Quercus marcocarpa).  This tree can survive for several centuries but is somewhat slow growing.  Its acorns and wide canopy are of course of significant value to wildlife.  The bur oak has a strong and deep tap root.  Tap roots have been measured 4.5 feet deep after only one growing season, even without the Waterboxx.   This tap root is likely the reason for the bur oak's drought tolerance.  This tree is a pioneer tree, meaning it is the first tree species to become established on the Great Plains prairie.  This tree can grow in zones 3-8.  It is very important if planting this tree to find a bare root specimen with an intact taproot.  Of note, if planting this tree from seed (a satisfying experience as oaks grow ~1 foot their first spring) be sure to follow the advice here to avoid planting a hollowed out seed.  You can buy this tree here.

Burr oak, Quercus marcocarpa, in the Indiana University Arboretum.  This tree, although slow growing provides excellent shade while being drought tolerant.
Another tree that is renowned for its beautiful canopy, the maidenhair tree (Ginkgo biloba), is also very long lived.  It grows 50 to 80 feet tall with a 30-40 foot spread.  This tree is dioecious (meaning the tree is either male or female, not both like most trees) and the female tree produces seeds which can smell rancid, so it is very important to only plant male specimens.   This tree is a living fossil, imported from China and having no living relatives.  This tree is also remarkably hardy, some even surviving the Hiroshima atomic blast.  It is also, of course, moderately drought tolerant once established (and luckily you will have the Groasis Waterboxx to establish the tree).  This tree grows in Zones 3-8.  These trees are a little pricier than most of the other trees mentioned here, but can be bought here.

A very well established Ginkgo tree in Bloomington, IN.  

How To Increase Drought Resistance Through Proper Planting

The manner in which trees are usually bought and planted tends to encourage drought sensitivity, not resistance, and makes the tree more likely to die during drought.  For the best drought resistance, trees should be bought bare root, not potted.  Potted trees (usually purchased from big box stores) have very poorly formed roots, and usually after planting these roots spread sideways.  This means that during drought (when the upper few inches of soil dries out) these roots will as well.

The most drought tolerant trees are bare root (never potted) when planted.  The best place to buy these trees is from the Arbor Day Foundation, which ships only bare root trees (very affordably too, usually one tenth the price of potted trees).  These trees may not look impressive at first, but bare root trees have much faster growth when established and soon surpass almost all store bought potted trees.  They also don't need staking as the root structure keeps them upright.

Secondly, we recommend a device called the Groasis Waterboxx PlantCocoon.  This device, available here, both stores and collects dew and rainwater.  It slowly releases this water to the roots of a growing tree.  This allows (and actually causes) the tree to grow deeper roots, making it more resistant to drought.  The Waterboxx never needs to be refilled after planting, and can be reused up to ten times.

Regardless of what tree you plant and how you plant it, remember the old proverb:  "The best time to plant a tree is twenty years ago.  The second best time is today."

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