How much water is enough?

How much should I be watering my trees? It's a question I get all the time. The answer is: it depends. Let's look at some of the details that will influence your tree's watering needs:

1. What is the species of the tree? This will inform us what its native growing environment would be. Our native trees such as Piñon and Juniper do not need much, if any, supplemental water if they seeded and grew themselves. If they were planted into the landscape then they will need some supplemental water. Some native trees are actually native to wet environments (also called riparian) such as the Dolores river valley or along creeks like McElmo. Although these trees are native, they need extra water to survive. The benefit is, they often grow very quickly. Examples include all poplar species (cottonwoods, aspens and poplars), all Willows, the New Mexico privet, native river hawthorn and river birch, etc. So it is important to know what the tree's native range is in order to determine what kind of watering regime it needs. 

2. Is it a “heritage” tree? By heritage we mean did the tree start from seed on its own and was probably there before the house was built. If the tree has been in the landscape longer than your house you may not need to water at all. However, building houses around existing trees can impact their root systems which means we need to help them with supplemental water. If your tree was planted in the last 5 years you will need more frequent watering since the tree does not have a large and spread out root system to find its own water. 

3. Is the tree “established”? By established we mean does it have a fully developed root system which will seek out and find more resources such as water and nutrients. If the tree was planted by the home owner but has been in the landscape many years it is probably established. Established trees usually need less water than newly establishing trees, but not always. Knowing the tree species will greatly influence how much to water an established tree.

4. How big is the tree? For obvious reasons this will greatly influence how much the trees needs watering.

5. The last question is what are the soil types and the conditions the tree was planted in?

So now that we know what kind of tree it is, where its native range is, whether it is “established” or not, and how big it is we can start to come up with a guess as to how much water it needs. 

Timing makes a difference

But before that we should talk about water frequency. Frequent watering can encourage trees to have shallow root systems. This is because the sweet spot for finding water and oxygen becomes very shallow in the soil. The problem with shallow root systems is they are vulnerable to drying out. If the water were to shut off unexpectedly the shallow roots would dry out quickly and die. 

Infrequent, deep watering encourages roots to grow deeper in the soil. This makes roots more resilient. It can also make the trees more anchored in soil. Deeper rooted plants can tolerate longer periods of drought in between waterings because they stay moist when the top few inches of soil become dry. It is important to understand that the soils need to dry out a little between watering. If you deep water your trees too often then oxygen can not proliferate to the deeper soils and they become anaerobic. Think of the smell of mud in a swamp. YUCK!

Soil makes a difference

Your soil type can also influence this balance between water and oxygen. Clay soils will hold onto water longer than sandy soils. We have pockets of sandy soils in a sea of clay soils in Montezuma county. 

Finally, soil compaction will impact not only your ability to water the tree, but it will impact the trees ability to explore the soils and oxygen availability in the soil. Generally, compacted soils are not ideal. Another blog should be dedicated to dealing with compacted soils. 

To summarize we want deep, infrequent watering, where the deeper soils stay moist and the top layers dry out in between waterings. 

But here's a caveat!

Now let's take all that knowledge and throw it out the window when talking about newly planted trees, which should be treated like babies. They are fragile and vulnerable and can’t handle infrequent and deep watering. They need constant water management. They need moderate soaks once to three times a week, depending on the species and planting environment. 

 

So how much water is enough?

Most experts in this area agree that 10 gallons per caliper inch (measured at groundline), once a week is adequate for most trees. For newly planted we want to water more often than once a week so I suggest 5 gallons per caliper inch two to three times a week.

Now that we’ve discussed all the factors that influence watering, let's throw out some numbers. It takes 620 gallons of water to water 1000 square feet one inch in depth. CSU Master Gardener guidelines suggest watering mature shade trees one to two inches every two weeks. That's 620 to 1240 gallons every two weeks. 

Most residential water systems will put out around 10 gallons per minute. To put out 1000 gallons of water on a tree the system needs to run for 1.5 hours. This may seem like a bit much, however the species of the tree and the soil types may allow us to scale back from there. For mature, established trees you have some wiggle room, but plan putting out a hose under the tree for at half blast for couple of hours at least once every two weeks. 

But what about watering systems?

Another common question I have is should I do a drip system or a sprinkler system?

A drip system can be great for newly establishing trees. It gets water exactly where you want it which is directly on top of the root ball. However, a drip system may discourage the tree from expanding its roots beyond the drip emitters. This is why it is recommended that you expand the drip system periodically in the life of the tree to encourage the tree to grow its root system. 

Sprinklers are less efficient at getting water where they need to go, however they do allow for the tree to expand its root zone and access not only more water, but more nutrients in the process. 

A common mistake is people who want to convert their sprinklers to drip. By doing this you are cutting off water to much of the root zone and redirecting it to only a few places. The fine roots of the tree will die until new roots can be put down underneath the emitters. Following this logic, converting a drip to sprinklers is ok. The existing roots should get water and new roots will be encouraged to grow into other areas. 

Some final thoughts

A common saying of mine is “trees are like humans, they do not like change.” The older the tree, the more it struggles with change. So if you are reading this blog and are thinking about adjusting your watering regimen, know that you should slow train your trees into the change you want to see. They will adjust their roots to new watering areas, new soil depths, now pockets of water and oxygen, but it takes time. It can take 60 days to grow a new fine, water absorbing root. Best of luck, and thank you for reading!