Many previously “welcomed” urban tree species have outlived their stay, becoming invasive and crowding out other plants in our Southern California landscapes. You can help by avoiding planting these trees identified by various sources (including the California Invasive Plant Council) to be too aggressive and habitat/resource-depleting for further planting.

Invasive Trees to Avoid Planting

invasive Tamarix ramosissima.jpg (do not plant this invasive tree in CA)

Athel (Tamarix aphylla)

Black Locust (Robinia pseudoacacia)

Blackwood Acacia (Acacia melanoxylon)

Brazilian Pepper (Schinus terebinthifolius)

Chinaberry (Melia azedarach)

Chinese Tallow Tree (Triadica sebifera)

English Hawthorn (Crataegus monogyna)

Peruvian Pepper Tree (Schinus molle)

Red Gum (Eucalyptus camaldulensis)

Russian Olive (Elaeagnus angustifolia)

Saltcedar (Tamarix ramosissima, T. gallica, T. chinensis)

Silver Wattle (Acacia dealbata)

Smallflower Tamarisk (Tamarix parviflora)

Tasmanian Bluegum (Eucalyptus globulus)

Plant These Instead

They are drought/heat resistant, low maintenance, and have no known significant pest or disease problems and are not currently overplanted). Find other suggestions here: https://www.cal-ipc.org/solutions/prevention/landscaping/dpp/?region=socal

African Fern Pine (Afrocarpus falcatus) (formerly Podocarpus elatior)

Cascolote (Caesalpinia cacalaco Smoothie®)

Desert Willow ‘Bubba' (Chilopsis linearis)

Fringe Tree (Chionanthus virginicus)

Island Oak (Quercus tomentella)

‘Maverick' Mesquite (Prosopis glandulosa)

Mulga (Acacia aneura)

Netleaf Hackberry (Celtis reticulata)

Pink Chitalpa (x Chitalpa tashkentensis 'Pink Dawn')

‘Red Push' Pistache (Pistacia x ‘Red Push')

Thornless South American Mesquite (Prosopis x Phoenix)

In your landscape, your trees and edibles should come first under drought and water restrictions! If there just isn't enough water to go around, your lawn and flowers should be sacrificed instead. Trees are our most valuable landscape resource and take years to maximize their benefits (shade, cooling, habitat/ecosystem enhancement, carbon dioxide storage, energy conservation, pollution filters, etc.).

Just a few slow, deep waterings with a garden hose away from the trunk and slightly beyond the drip line will keep established trees alive, even during summer.  Remember to keep trunks dry!

Here are some other tips:

• Spread and maintain 2-4” of mulch around garden plants and trees (3-4” for wood chips, 2” for pebbles, decomposed gravel, etc.) keeping it a few inches away from tree trunks. (Note: dark colored dyed mulches should be avoided in inland and desert areas due to their high surface temperatures that can > 60 degrees F hotter than living plants and lighter colored mulches.) 

• Water early in the morning when soil evaporation is minimal.

• Control weeds. They compete with other plants for water.

• Avoid fertilizing. Nitrogen increases growth and the need for more water.

• Don't plant new plants during the summer when temperatures are highest. Even drought-resistant native and non-native plants need regular watering their first season.

During the drought, your trees should come first! Because many well-maintained trees don't reach their prime until their 4th or 5th decade, it's important to keep them watered during the drought and water restrictions. Older trees absorb higher levels of carbon dioxide, do a better job of cooling urban heat islands and providing shade. They also filter air and water pollutants, reduce soil and water erosion, provide habitat for wildlife, enhance privacy, beautify neighborhoods and parks, enhance property values, and even improve mental and emotional health. Trees are worth saving! The price of starting over is just too steep. 

Recognizing early signs of drought stress is important because irreversible damage can occur that no amount of watering will correct. If you cash in your grass for a water district rebate, remember to water any trees that relied on water from the lawn sprinklers. This is because tree roots grow laterally quite a distance, often well beyond the dripline of the tree under the lawn. Over time, start watering them more deeply and less often to encourage deeper roots and enhanced stability.

Just a few deep waterings in mornings or late evenings with a garden hose during spring and summer can keep a tree alive, even during a drought. Keep the trunk dry and water from the mid-dripline outward. Apply a slow steady stream of water, moving the hose to another section of the tree every 1-2 hours. Another inexpensive way to water your trees is to encircle them with soaker hoses that connect to a garden hose. The soaker hose should be in concentric rings about one foot apart starting two feet away from the trunk for a larger tree, closer for a more recently planted tree with a more confined root system.

Check trees regularly for these common symptoms of water stress:

• Wilting or drooping leaves that don't return to normal by evening

• Yellow, brown or sometimes gray leaves that may drop from the tree

• Small new foliage and stunted overall tree growth

Other ways to reduce tree water loss include:

• Keep weeds out (they compete for water

• Maintain a 3-4” of organic mulch or 2” layer of inorganic mulch (pebbles, decomposed gravel) around your trees, starting a few inches away from the trunk outward.

• Avoid fertilizing since that increases their water need.

• Do only necessary pruning (to remove dead wood and any dangerous limbs that look like they might fail) since this stresses the tree and can increase its water need.

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UC Resources at Your Fingertips:

Free Download Publications: https://anrcatalog.ucanr.edu

-Keeping Plants Alive Under Drought and Water Restrictions

-Sustainable Landscaping in California

-Lawn Watering Guide for California

-Use of Graywater in CA Landscapes -

CA Institute for Water Resources: http://ciwr.ucanr.edu/ (blogs, climate-smart ag, podcasts, etc.)

UC Cooperative Extension Master Gardener Program:

For more drought tips and help with your home gardening questions, contact a UCCE Master Gardener volunteer. Find your local program atucanr.edu/FindUs/ 

Most trees in California need supplemental irrigation above and beyond what Mother Nature supplies naturally. Even drought-resistant species need regular watering through their first growing season due to their shallow roots. Once trees become established, it's important to water less often but more deeply to encourage deep rooting and structural balance above and below ground. Both under and overwatering can lead to unhealthy trees and even death if the situation is not corrected. Trees receiving too little or too much water exhibit similar symptoms since, in both cases, water is not available to the plant. Trees initially wilt, grow slowly, and develop yellow leaves. Over time, growth stops and leaves become brown and drop. Overwatered trees often develop lower crown and root rot from one or more disease-forming pathogens.

Knowing what type of soil you have (soil texture) is as important as knowing the water needs of your trees. Use the ‘feel test' (pictured below) to find out how much water your soil holds and how often to water. Heavier clay-based soils hold water longer and drain more slowly than sandier soils that need to be watered more often for shorter periods of time.

do a feel test of your soil to help you know when to water

Trees should not be watered on the same irrigation system used for lawns and groundcovers. Soaker hoses and drip systems allow trees to be watered less often but for longer periods of time than your lawn or groundcover. Avoid applying water too close to the trunk. Instead water half-way between the trunk and the dripline of the tree and outward. If you use a garden hose, apply the water on the lowest volume possible slowly, moving the hose every few hours to each of four quadrants around the tree. 

Applying a 3-4 inch layer of mulch around the tree can reduce soil evaporation.  Use only non-flammable mulches in fire-prone areas within five feet from the house and non-contiguous for the first 30' away from the house. In all cases keep mulch a few inches away from tree trunks to keep the trunks dry.

Tip: Before planting a tree, make sure there is adequate drainage. Dig a hole where you want to plant it (the same depth of the pot, which is about one foot) and fill the hole with water. Let it completely drain and refill it. Measure the time it takes to drain one inch using a ruler. If it does not drain more than one inch an hour it is not a good location for your tree. Avoid adding compost or soil amendments to try to correct the problem since tree roots will likely grow in circles, staying within the confines of the amended hole rather than growing outward the confines of the amended hole rather than growing outward.

do a quick drainage test before planting a tree