Watering Street Trees in Santa Monica Watering trees is extremely important to their growth, health and ability to produce the important environmental benefits that we as a community rely on. However, like anything else, too much of a good thing can be detrimental. By following some basic guidelines you can have healthy street trees and conserve water at the same time. FUNDAMENTALS OF WATERING STREET TREES The species, age of tree and type of soil have a bearing on how much to water. o Some types of soil hold water longer than other types. o Some types of soil dry out quicker than other types. o Watering young trees is different from watering mature trees. o Some species do not require as much water as others might regardless of age. See Attachment 1 for a list of watering needs of individual species. To find out what species your street tree is contact the Community Forester at [email protected]. o The seasons and weather conditions influence when to water. Checking soil moisture content throughout the year will tell when a tree needs water. o Surface grade dictates how fast the soil can take the water. Apply too much at once or too often and you get surface runoff or puddles. KNOW THE TYPE OF SOIL It is important that sprinkler systems are properly designed and maintained so they apply water at a rate low enough to allow all it to infiltrate into the soil without running off or creating puddles somewhere it’s not supposed to. The rate at which soil can absorb or take in water is the infiltration rate. The rate at which a sprinkler system applies water is called the application rate. If the application rate is higher than the soil's infiltration rate surface runoff is achieved, resulting in water collecting on the surface and/or flowing away from where it was supposed to go. This wastes water as it doesn’t provide the tree with the water it needs. This condition can be offset by applying repeating shorter water cycles that gives to sol time to absorb the moisture in between cycles. The irrigation system shown in the photo at left shows a front yard landscape with a steep grade down to the sidewalk. The system has been running long enough to pass the point of runoff and the water applied from this point forward is wasted. To avoid this problem the system should be set with short cycles and long intervals in between each cycle. This allows the soil to absorb the water as it is applied. In order to have an idea of the infiltration rate it helps to know the type of soil. A general rule to follow when applying irrigation is: sandy soil requires water on a more frequent basis and clay soils require less water on an infrequent basis. An easy way to determine the type of soil: Dig down about four inches and take a sample of moist soil about the size of a marble. o If it can be rolled up into a tube thinner than a pencil and hold its shape, then its clay soil. o If the soil breaks apart easily and has a consistent, gritty texture, it is sandy soil. o If the soil contains a lot of organic matter of varying size and consistency, it is loam. Loamy soil can also vary from clay loam or sandy loam. Loamy soil that is easy to break apart into small particles is sandy loam. Loamy soil that breaks apart into large chunky particles is clay loam. Soil with the right amount of water should be moist but not saturated. A simple test to check moisture content is to take a handful of soil and squeeze it tightly then release the pressure. If the soil retains its shape and is not sticky or oozing water, the moisture content is favorable. Another consideration is the seasonal changes in weather. During spring and fall, the days are shorter and cooler and the soil temperature drops, and trees need less water. As the days get longer and warmer, trees need more water. As the seasons change, the frequency of watering needs to be adjusted accordingly. This is usually done by reducing the number of days between each time you need to water and reducing the amount that is actually applied to the soil. If no adjustments are made throughout the seasons, over- watering or under-watering occurs, which in turns leads to poor tree health. GUIDELINES FOR WATERING STREET TREES Check the Community Forester’s list of water requirements for street trees to get an idea of how much water your street tree needs. (Attachment 1) Key factors to remember when watering street trees: o Species There are over 225 different species of trees in Santa Monica and many of them require different amounts of water at different times of the year. Santa Monica’s forest is primarily made up of trees that flourish in a moderate climate with naturally dry conditions. Many species found in Santa Monica do not require regular water and actually thrive when they receive minimal amounts. Other species require no irrigation during the summer months as it is detrimental to their health. Mature or established trees usually can get by with infrequent watering depending on the species of tree. Most of Santa Monica’s street trees require a climate with long, hot summers and mild winters with a moderate amount of rainfall. They do best in sandy loam soil but can tolerate other types of soils. The most important area to water for deciduous trees is within the dripline (from the trunk to the outer edges of the trees branches). For evergreens, water 3-5 feet beyond the dripline on all sides of the tree. o Current age and size Older trees are sensitive to increased amounts of water. Trees that have been thriving on a certain level of soil moisture and then subjected to increased amounts can respond in different ways. Increased shoot growth adds new foliage and weight to the limbs in the canopy and increases pruning requirements. In order to reduce the potential for limb failures pruning cycles need to be increased. Too much water can also kill roots from lack of oxygen in the soil. Mature trees that suffer from too much water in the soil will respond with dead tips, dieback throughout the canopy and excessive leaf drop. In extreme cases, trees with dead roots can fall over. The tree in the photo at right is a mature drought tolerant tree that has a sparse canopy and chlorotic foliage because of regular watering. This is evident by the dieback in the tips of the tree seen in the photo at left. The photo at right shows the trunk base clear of turf. This helps prevent damage to the trunk from water, lawn mowers or line trimmers and maintains a strong healthy trunk base. Water on the trunk can lead to decay if there are wounds at the base of the tree. This eventually weakens the trunk and creates a potential for total tree failure. Water on the trunk creates conditions for decay to progress when there are wounds at the base of the tree. There are several fungal diseases known to afflict trees that thrive in of southern California’s climate. The diseases that infect these trees can cause considerable damage and even the demise of a street tree. These problems mainly arise due to conditions that are different from their native habitat. Conditions such as summer watering or excessive watering in the winter result in fungi, bacteria and viruses in the soil that trees cannot withstand or compete with. This condition can ultimately lead to the failure of a mature tree in spite of a healthy appearing canopy. The photo at right and below show the fruiting bodies of two fungi that are commonly associated with trees that have been wounded and suffer from internal decay. Improper or excessive watering at the base of a tree creates conditions for these fungi to grow and accelerate the decay process. When trees display these fruiting bodies it is an indication of extensive internal decay. Report a tree with fruiting bodies of fungi. [email protected] The main thing that new trees need to become established is a source of moisture. Regular moisture allows a new tree to grow quickly by developing a consistent amount of foliage and shoot growth, while irregular watering forces the tree to only produce the foliage that it can sustain on minimal or infrequent amounts of moisture. o Young trees that have been recently planted in the ground need approximately 10 gallons of water every two weeks. o A simple method to follow is to fill up the swale (shown in the photo below left) around the trunk base with water and let it percolate down into the root ball. After it has all drained into the soil, fill the swale up once more and allow it to drain. Follow this method twice a week during warm months of May through October and once a week during the cool months of November through April. When watering trees it helps to understand the following facts about roots. o A tree’s root mass is its foundation and provides the structural support it needs to remain standing upright. Over watering leads to damage to that foundation and can affect the stability of a street tree. o Most if not all of a street tree’s roots are found in the upper 2 feet of soil.