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Thirsty Plants and Watering in Times of Drought

By Judy Quan

Extended drought

California has been in a drought cycle for two years and we appear to be entering our third consecutive year of drought. We cannot depend on the winter rains and storage of melted snowpack to provide enough water for unlimited residential irrigation. We will likely see mandatory restrictions for watering. The weather is changing to include warmer, more prolonged heat. I have been thinking of my garden and what care it needs during an extended drought. Our soils are very dry and our plants need more support.

A garden and thirsty plants

Front yardjpg
My garden is located in Albany, CA, where it rarely gets to freezing and the weather is very much moderated by the ocean breezes. The 2012 USDA plant hardiness map divides North America into 13 hardiness zones based upon average annual low temperatures, and I am in USDA Plant Hardiness Zone 10a with average minimum low temperatures from 30°F to 35°F. Of 24 climate zones defined in the Sunset Western Garden Book, I am in Sunset Zone 17. Sunset zones encompass more elements of what affects plant growth such as the length of the growing season, rainfall, the winter lows, the summer highs, wind, and humidity. Learn more about climate zones and why knowing your climate zone is important.

I have gardened here for years and have a clay-soil garden that consists of a mixture of plants in the ground and in containers. I tend to be a more relaxed gardener who is interested in fragrant, flowering ornamentals and perennial edibles such as citrus, figs, and blueberries. I love trees. Many of the plants in my garden were chosen and planted years ago when drought tolerance was more of an after-thought. So I have some long-time thirsty plants in my yard.

Thirsty plants, a different way of watering. For those of you who, like me, have more thirsty plants in your garden, what can we do to help our plants survive and perhaps even thrive while conserving water? Our plants need time to learn how to adapt to a different way of watering.

Learn the watering requirements of the plants in your garden, especially your favorites.

Figuring out what you love most helps you to make sure that the needs of those plants are met by learning to water them effectively.  For instance, for me, it turned out that the plants I loved most were mainly trees (young and old), my citrus (in the ground and in containers) and my mom’s rose planted many years ago. These plants turned out to have diverse water needs.

Just how thirsty are your plants?  In my garden I have trees with widely differing needs based upon age and type of tree. In your garden, you can find out more about the water requirements of your trees by looking at the SelecTree A Tree Selection Guide, and Water Use Classification of Landscape Species WUCOLS IV. WUCOLS IV provides information on the water irrigation needs of plants so that one can match the water supply to the needs of the plant, giving information necessary to avoid overwatering beyond the needs of the plant. 

Young trees (1-5 years in the ground) need more frequent watering.

Young Red Maple Tree © Judy Quan
Check the water needs of your tree. I have a young street tree, a red maple, Acer rubrum “October Glory”, that has been in the ground for about 3 years. This tree is a thirsty type, and when mature, the maple will need to be watered monthly. Canopy.org has a lot of good information on the water needs of different trees with a very interesting webpage on watering guidelines

Take care when watering near native oaks.

Studies have shown that California native oaks may benefit from supplemental summer irrigation during prolonged drought even though they are relatively drought tolerant. However, they are also easily over-irrigated… 

Water young trees once a week. Young trees (1-5 years after planting) need more frequent watering than mature trees, but they need a smaller volume of water. In hotter interior areas, a young tree may need at least 10-15 gallons of water per week, depending on soil and climate, but in a moderate coastal climate such as Albany's 5 gallons a week is likely sufficient. 

Water slowly and deeply. The roots of a young tree are mostly located near the trunk and grow 12-18 inches below the surface. Slowly soak the area near the base of the tree each time. Deep water your tree to encourage roots to go down. Let the soil dry out a bit before watering. Water a little more frequently when the weather is hot and dry. Water your trees regularly and apply the water slowly enough to permeate the soil down to at least 18" to 2'.

Transition to more infrequent watering as the tree matures. As the tree matures in years 3 to 5, my tree likely will need to be watered at least once a month 20-30 gallons. The monthly soakings will maximize growth while conserving water. 

How do I know how much water I am applying? 

Hose Water Flow Meter Example
Knowing how much water you are applying helps you to meet the water needs of your tree and be water efficient. Canopy.org recommends regularly watering 10-15 gallons/inch of trunk diameter, though the frequency is dependent on whether the tree is a low, moderate, or high water needs species. A 5-gallon bucket or a flow meter can measure the volume of water you are applying to the tree, thus giving you an easy way to manage the irrigation needs of your tree.

  • Fill a 5-gallon bucket to measure the volume of water. 
  • You can use a flow meter to measure the amount of water distributed by your hose. 

Established Trees

The Arbor Day Foundation suggests a rule of thumb for watering established trees: 10 gallons of water (equivalent to a 5-minute shower with a low-flow head) for each inch of the tree’s diameter. How often you should water will depend on the size of your tree, soil conditions, and weather conditions. 

Most established trees do not need irrigation in the rainy season (if it actually rains during the winter months), and they only need supplemental watering once a month in the summer and during winter months with little or no rain. Trees need water if the weather is extremely hot or windy. Water before, during, and after our Diablo Winds, which are like the Santa Ana winds, to reduce stress. For sensitive trees like citrus and subtropicals, water before cold weather to minimize frost damage. Watch the weather to adjust watering accordingly.   

Trees sensitive to wind, heat, and sun

50 Year Old Maple Trees
The 50-year-old mature maple trees in my yard need watering further away from the trunk since the roots of the mature tree extend further than the canopy of the tree. Adding water near the trunk of the maples will not reach the roots. Slowly soak the area throughout the root zone for several hours to reach the roots 12-18 inches below the surface. Trees need deep, infrequent watering. Most absorbing tree roots are in the upper 18 to 24 inches of the soil, under the tree, and beyond the edge of the canopy (also named drip line). The maples are very sensitive to heat and wind; so I need to be vigilant about watering when excessive heat and high winds are predicted. 

A note about sunburn. We pruned our maples over the last 4 years to pull major branches back from our neighbor’s yard. The major branches provided protection for the entire tree from the sun. Maples have thin bark that when exposed to sun and heat results in sunburn and damage to the tree. To properly”moisturize” and hydrate the exposed bark, we water more when the temperatures are warmer. 

Trees with moderate or thirsty water needs

Michaelia front of house with house number near window
The Michelia champaca, now re-named Magnolia champaca, is the centerpiece of my front yard. The Michelia is an evergreen tree with powerfully fragrant flowers that reminds people from the semitropics of a home faraway. The Michelia Champaca is a native of southeastern Asia and flowers mostly in summer and late autumn. For years, I did not water this tree enough and it looked like it was dying and needed to be replaced.  Since it has received more water, the tree has regained many leaves and flowers. I know now that it is a tree that needs regular water; so I need to make sure I am watering it more frequently, with deep, slow watering. I have decided to get a splitter for a soaker hose and put it on a timer to make sure my favorite tree gets the water it needs in the way that helps it the most.

Citrus in the ground

Mandarin Tree © Judy Quan
Citrus trees need adequate soil moisture during spring to set fruit and steady water in summer and fall to produce good fruit.

Ben Faber, University of California Cooperative Extension (UCCE) Farm Advisor, in Irrigating Citrus with Limited Water,  says that “as an evergreen in California’s Mediterranean climate, with wet winters and dry summers, citrus requires some water all year long”. During the drought when we have not received much winter rain, citrus must have supplemental water.

According to Faber, we can regulate or reduce watering to citrus while minimizing the impact on fruit production if we understand when our citrus is most in need of water. Water stress during flowering increases fruit and flower drop, and water stress during rapid fruit growth reduces fruit size. Additional water might be needed during the flowering period, but during fruiting, it is essential to avoid water stress when the fruit is growing. For navels and mandarins, it is possible to identify the critical periods when water is required since flowering is separate from the fruit set. In other citrus cultivars; however, the critical periods may overlap. For example, Meyer lemons can have flowers and fruit in all phases at the same time.  So, for me, I will really tune in what phase of growth the different citrus plants are in, watering during times of bloom and fruit growth.   

For best crop production, citrus requires some fertilization with a citrus-specific formula that includes nitrogen, which should be applied just prior to bloom in January or February, then a second application given in May and a third application in June. Just remember that new growth also means that the plant will require more water. For more information on citrus, see http://homeorchard.ucanr.edu/Fruits_&_Nuts/Citrus

Citrus and Other Plants in Containers 

Container Garden © Judy Quan
Containers must be watered consistently and adequately because they can dry out quickly. Watering requirements will vary according to the season, type of container, and sun exposure. 

Containers need to be checked daily.  Use your finger to monitor the soil moisture. Stick your finger into the soil a couple of inches and if it feels dry, water. If it is moist, don’t water. Water until some runs out the bottom of the container. Check that the water is reaching the roots and not just running out the sides. On hot or windy days, containers may need daily watering. 

Never let a container dry out completely. If it does, the soil may become hydrophobic, repelling water, and consequently, water may run through the container without actually moistening the soil. It is difficult to re-wet

Container-grown plants may have different fertilizer requirements. Some need frequent, light fertilizing to grow well. As we water, nutrients are washed out of the soil and need to be replaced. One can include a light dose of water-soluble fertilizer with each watering or one can put a slow-release dry fertilizer in the potting soil. Citrus requires more fertilizer when container-grown. Non-edible ornamental plants in containers may not require much or any additional fertilizer.

I may put in a drip irrigation system to ensure regular water for the containers since the containers require so much attention even in such a moderate climate as mine.

Roses

Rose © Judy Quan
Roses too need deep watering at 3-7 day intervals, but where I live the interval could be stretched out further. Shallow watering leads to more surface roots and the need to water more frequently.  Roses with a 3-foot canopy across might need about 3 gallons of water per week. These numbers should be adjusted for the weather and the particular site. Roses too can be encouraged to develop deeper roots and thrive on less frequent watering. Let the rose tell you when it is thirsty.

Apply water within the dripline of the plant and water slowly and deeply. Forget the fertilizer for now during spring and summer to reduce growth that would increase the need for water. Keep the leaves to help keep roses cool during summer heat. Cut back watering for dormant roses; they need just enough to keep the soil moist.

You’ll know if you aren’t watering enough. Roses react to drought by dropping leaves, decreasing flower size, or showing signs of early dormancy. 

The devil is in the details! What to know about watering.

How to Know When You Have Watered Enough

Learn how much water is needed for deep and slow watering. Time and check the water flow to see when the soil is actually wet down. Check the soil to determine when to water again by digging in the soil 6-8 inches below the surface. If the soil is dry, add water. If it is wet, let it dry before watering again. The bulk of active roots are in the top 2 feet of soil. You need only to initially time watering to get a sense of how long to irrigate to get the soil wet in the top 18 inches.

According to the Community Forest Advisory Board of San Diego, deep watering replenishes the water-holding capacity of the deeper soil layers and enhances tree resilience during periods of drought. If your soil is extremely dry, you will have to build up moisture in the top layer of soil before you are able to recharge to deeper layers. Water will only move down the soil profile under saturated conditions, so this requires the wetting of the shallower soil layers prior to the deeper layers being wet. Once the top layer is hydrated, you can deeply water for long run times—hours not minutes— and slow application rates. 

Tip: Add a root irrigator, a watering tool that is connected to a hose and that can be inserted into the ground to water soil more deeply underground, or an olla, a water permeable, low-fired clay ceramic vessel that is buried and then filled with water periodically, for deeper watering.

Consistency in soil moisture is important. 

Allowing the soil to completely dry out — or allowing it to stay soggy — will result in plants that are stressed and therefore more prone to disease and infestation. Keeping the soil consistently moist is essential to healthy plants. 

Apply water when transplanting new plants.

A common mistake is to underwater new ornamental and edible plants and overwater established mature plants. New transplants, even native plants,  need more frequent and shallow irrigation than established plantings. The rootball of newly planted plants should never dry out. Dried-out root balls become aquaphobic, meaning they repel water, which makes it difficult to re-wet the rootball. When planting, first dig the hole for the new plant, fill the planting hole with water. Allow water to soak in, and then plant, and water thoroughly. This will give the rootball an easier transition.

Know the watering techniques and the water needs of your individual plants. 

  • Know your plants. Water plant and record date; watch plant for signs of water stress in citrus, roses, and other plants -- rolled leaves, slight cupping of leaves; record date and the number of days since first watering; water one day earlier. Adjust for temperature and precipitation. Look at containers daily since they are so sensitive to underwatering.
  • Be aware of the past errors in watering habits and change them slowly.  For instance, because of hand watering with a hose, I have made the mistake of watering everything in the ground and in containers every time I water. Some plants have grown used to frequent shallow irrigation. If I suddenly stop watering so frequently, the plants, even the big ones like the trees, can suffer because of a sudden loss of a source of water for shallow roots. So I need to change my habits slowly over several weeks, adjusting for frequency and for weather…. And if you have trees planted in lawn areas, don’t forget to water the trees even if you stop watering the lawn. The lawn trees may need time to adjust to the change.
  • Conserve water by mulching, mulching, and mulching. Apply and maintain a three to four-inch layer of mulch around garden plants and trees to keep water in and the weeds out. Be sure to maintain mulch-free area at least a foot away from tree trunks to avoid wet trunks and crown, which can be subject to disease-forming pathogens.
  • Control weeds.  Weeds usually outcompete garden plants for water.  Pull them when they are small.

Last Words – Walk Your Garden

The best advice I ever got was from another Master Gardener who told me to walk my garden every day.  I don’t do it every day, but I try to do it often and notice how my plants are doing. Walking your garden helps you to really know your garden. Look for weeds. Pulling weeds gets you closer to the plant and to notice and feel your soil. Notice what new growth (leaves, flowers, fruit) there is. Look for yellowing, droopy leaves, or things that show need to be fertilized or pruned.  Look for what needs to be cleaned up or mulched. Notice when plants need water; see how they look in the morning before the heat of the day. Look closely at the container plants and feel the soil for moisture in containers and for good soil texture.

Questions about your garden?

Still Need Help? Would you like more information or help with other gardening issues? Email us at acmg@ucanr.edu. Or contact us through our website http://acmg.ucanr.edu/Contact_Us/ .

Resources

Take care when watering near native oaks

Studies have shown that California native oaks may benefit from supplemental summer irrigation during prolonged drought even though they are relatively drought tolerant. However, they are also easily over-irrigated leading to fungal diseases such as oak root fungus (Armillaria mellea) and crown rot (Phytophthora spp). Letting the soil dry out some between irrigations will help prevent these diseases. Water should be kept at least ten feet away from tree trunks and most should be applied in the outer two-thirds of the root zone which may extend two to three times beyond the canopy of the tree.

“Keeping Plants Alive under Drought or Water Restrictions” 

https://anrcatalog.ucanr.edu/pdf/8553.pdf

Keeping Plants Alive under Drought or Water Restrictions ….. And Planning for the Future by Janet Hartin, Ben Faber, Loren Oki, and David Fujino https://ceriverside.ucanr.edu/files/218056.pdf

Tree Preservation and Watering Under Drought Conditions - Community Forest Advisory Board, City of  San Diego. (This link has extensive handouts with great illustrations and photos. The document is part of the information available from the California Center for Urban Horticulture at UC Davis) https://ccuh.ucdavis.edu/sites/g/files/dgvnsk1376/files/inline-files/Tree%20Watering%20Handouts.pdf 

Arborday.org and watering trees https://arbordayblog.org/treecare/how-to-properly-water-your-trees/ 

Save Our Water, Save Our Trees https://saveourwater.com/trees

Sacramento Tree Foundation https://sactree.org/tree-care-tips/how-to-water-trees/ 

Canopy - Healthy Trees, Healthy Communities https://canopy.org

Albany CA - Urban Forestry - Street Trees Information, Maintenance, and Care https://www.albanyca.org/departments/public-works/urban-forestry 

Irrigating Citrus with Limited Water by Ben Faber https://anrcatalog.ucanr.edu/pdf/8549.pdf 

Container Gardening and Watering from Santa Clara Master Gardeners https://mgsantaclara.ucanr.edu/garden-help/container-gardening/#watering_for_containers

Sunset Climate Zones https://www.sunsetwesterngardencollection.com/climate-zones/zone/san-francisco-bay-area-and-inland

Mystified by Climate Zones?  blog by Butte County Master Gardener https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=21293

What is my climate zone https://cagardenweb.ucanr.edu/Your_Climate_Zone/ 

Roses and Drought from Tulare/King Master Gardener  https://ucanr.edu/datastoreFiles/268-624.pdf

Keeping Roses Healthy in a Drought by Nanette Londeree, Master Rosarian, Marin County Rose Society https://www.rose.org/single-post/keeping-roses-healthy-in-a-drought

Marin Rose Society “Roses in Drought Conditions” https://www.marinrose.org/care-basics/roses-in-drought-conditions/

Pacific Rose Society “Growing Roses During the Drought” http://www.pacificrosesociety.org/RoseDrought.html