Watering Tips: including deciduous and evergreen woody plants

Text Copyright 2002 - 2009 by Mario D. Vaden

No matter where we live in Oregon, similar principles apply for watering. I've worked in Portland, Seaside and Medford, observing proper and foolish irrigation methods in each area - rainfall averages ranging from 19" to 75" per year. In each region, plants benefit from mulching and proper irrigation; because each area can experience consecutive weeks without rain.

I've seen a lot of people who water during the hot part of the day, plant woody plants without mulch or believe the myth that native plants are more drought tolerant. The truth is that drought-tolerant plants are drought tolerant, not native plants. Some of our native plants need plenty of sprinkling.

With newly planted woody plants like conifers, shrubs and other, it's easy to remove mulch to check moisture of the soil. But some people don't have skill or experience to estimate moisture content inside a rootball, which the most important soil to keep moist and watered.

To the left is a photo of a dowel rod inserted in a root ball to monitor soil moisture. That homemade tool provided success for customers who did their own irrigating. Its my preferred method too. Saw 1/4" to 1/2 " diameter wood dowels into 12" to 14" lengths, sharpen an end, and insert one, two or three of these into a root ball.

The sticks are left in place and pulled out every day or two to see if the wood surface is moist or dry, indicating the presence or absence of moisture. This method can be more practical than trying to fiddle with moisture meters, especially since the soil textures in rootballs vary from clay to porous compost. Moisture meters can work too, but consider the dowel method for a good alternative.

The sharpened dowel method is an inexpensive and reliable means to check the concealed area within a rootball. As long as the stick still fits tight, try and use the same holes each time. This limits probing in the root zone - an insignificant effect on the roots.

One extra benefit of the dowels is lack of malfunction. What do you do if your moisture meter malfuntions? How would you know if its defective? That's the best part about dowels. They tell the truth about the moisture. That stick will be moist, or it will be dry.

A moisture meter could be used with the dowels, but I see no need. It's not hard to tell if a wood dowel is dry, a little bit dry, moist or saturated. Also, a dowel can reach deeper than a finger. For small plants, shorter probes are needed.

A newly planted woody plant and its rootball are comparable to the wick of an oil lamp and it's oil reservoir. Pretend that the leaves are the flame, the stem and roots are the wick in the oil, the reservoir is the rootball, and the oil is water in the rootball.

An oil lamp wick does not receive oil from any other source than the reservoir. Likewise, a newly planted tree can't draw water from fill soil, but only from within the rootball that its roots are contained in.

Suppose you set an oil lamp in a wide pan filled with extra oil. Even if the pan had 10 gallons of oil, the lamp's wick would be limited to the oil in the small reservoir. The excess oil in the pan would be irrelevant, unless you could add it to the small reservoir.

Plants and rootballs are very similar. Although some water can slowly move from fill soil into the rootball, the important thing to remember, is that the roots of newly planted trees are only within the rootball. And the roots only draw water from within the rootball. The surrounding soil could be moist, but it the rootball dries out, the plant can die. The root ball is the most critical area of soil to keep moist. Eventually the roots will grow and extend into the fill soil, but that may take weeks or months.

This does not mean its unimportant to keep the fill soil moist too. That is important also, because the fill soil is where new roots growth will extend to.

The idea for irrigating is to keep roots wet or moist, not drenched or water-logged. Remember that a lot of native soil from a planting hole in Oregon, is slower draining than the fill soil or potting soil of nursery plants. Don't water until the plant is sitting in liquid.

The problem for many people is not how frequently they water, but that they don't monitor the amount of water or level of moisture in the soil.

Barkdust will make or break success in most cases. Maintain a consistent 1" to 3" deep layer over the planting area and rootball. That will help retain moisture - read our advice topic on mulch and barkdust.

If you plant in the fall, don't forget to check the moisture of the rootball in late fall or winter if there is no rain for 2 weeks or more. Or if there is just a quick light drizzle of rain that barely penetrates the soil.

In the spring or summer, water new shrubs with 1 gallon to 3 gallon size root balls almost every second to third day when weather is 60 degrees to 75 degrees. If the weather is 75 to 85 degrees, watering may be needed every day or two. If the weather is 85 to 100 degrees, waterering could be needed daily. In the high 90s, irrigation could be required twice per day if the tree has a large canopy of foliage ans a disproportionately small rootball.

But you don't need to flood the plants. Suppose it's a 100 degree day, and you have a 6' tall arborvitae - newly planted. Often, those have a very large amount of foliage with a proportionately small rootball. If you water it twice during the 100 degree day - say with 2 gallons of water - it's fine to apply 1 gallon in the morning and another 1 gallon at night. You can even spray part of that gallon into the canopy so some of it slowly trickles into the mulch.

How will you know if that was enough or too much? Simple by monitoring the moisture. That's why formulas are hard to make, and probably not good to follow. The only smart formula is to monitor moisture in the soil on a regular basis. In a week or two, most people will begin to figure out how fast their soil dries out.

Some factors will change watering suggestions. One is wind, the other is the size of the canopy. Humidity can reduce the irrigation requirement a little bit.

Even given these suggestions, you should still be monitoring the moisture in the soil on days that you are not watering.

It's better to monitor the water in the rootball and backfill soil, than it is to monitor water loss or stress by waiting to see wilting leaves. Some people believe that it's better to look at foliage for signs of drying - then water. I don't recommend that.

Letting roots get too dry is not good. Any living plant needs moisture. There is a point where the moisture drops so low, that the plant can't recover. This is called the permanent wilting point. Since plants always need a little moisture, the rootball needs to be moist at all times. So where is the danger point? You don't want to find out! Keep the rootball moist, but not saturated.

The most important area to water is the rootball area under the canopy of the tree. Many people loose plants because they water the area surrounding the perimeter of the planting hole and surrounding soil only, but skip irrigating directly above the roots and root ball. It's good to soak the perimeter area too, but be sure to irrigate the root zone. Sometimes, I spray manually into the canopy and let water rain down. And I'm a firm believer in using occassional hand watering even if there is an irrigation system. If you rely 100% on automatic irrigation to do the job, odds are that you will be sprinker watering some plants too much.

One of my favorite watering tools is a long wand with a shower head at the end. It allows me to reach and irrigate into flower beds where I can't step, or over tall hedge plants like 8' arborvitae.

For trees, it's been said, that it may be better to water every few days or weeks - if established - at a slow irrigation rate for long intervals. My view is that irrigation rates should be reduced for many established trees so the roots may go deeper. If the species are small like Japanese maple or vine maple, maybe irrigate more often. But with large conifers like Douglas fir or western hemlock, why not eventually drop the irrigation level to near the lowest moisture levels that could occur naturally. Putting aside drought year rainfall averages.

That bit of advice is for established plants. What about newly planted woody plants? Professional research indicates that newly planted nursery stock can establish better if watered frequently to keep the rootball and roots moist - not water-logged. That would apply for the first growing season or two. After that, less irrigation may be needed. It depends on where you live. And if the mulch is thick - like 3" to 4" - that will reduce watering needs too, once the mulch is moistened.

I read an article once that may have been a bit misleading. It said that roots do not grow DOWN TO water, but grow WHERE water is. From a technical point of view, that may be accurate, but to the common understanding of an average person, that may be more confusing than helpful. Yes, roots do grow where there is moisture. But as the moist "soil layer" progressively "descends" downward with seasonal drying, so will the roots also, which have been growing in it and following it along. They can "follow" due to the way they grow and extend at the same time that the moisture area moves downward. So figuratively, and practically speaking, it is safe to say that roots grow down to water. And the lower that roots can grow (or be trained down to), the better the plants will be anchored. That's why it's good to keep plants moist, but not flooded.

Years ago, I read a book showing how someone made plant roots extend by "training" the root system. A small obelisk rock, with a flat-cut base, was set in a very shallow tray pot, like a Bonsai pot. A plastic cylinder wrap was used to hold soil around the stone, up to its top. A small Chrysanthymum was planted on the very top. As the plant grew, the plastic was peeled down and soil removed and washed away. The plant's roots elongated more, and more and more, extending more than one foot across the face of the rock. Eventually, the finished project was a plant on top, with bare smooth roots grasping the rock, and extending into a couple of inches of soil in the bottom of tray to get moisture and nutrients. This is why I indicated that in general terms, roots can grow to water..

And, slightly reduce the level of irrigation once plants are established, even to the point of a tiny bit of dryness. Or maybe better said, to a tiny bit of moisture.

If you have planted something like arborvitae in summer that have enormous canopies compared to the root ball size, inquire at a local nursery for an antidessicant spray. That kind of spray seals the foliage to retain moisture. It's a secondary measure. It's best to prolong planting of plants like that until weather is cooler, but in case the planting occured anyway, that spray is very effective on plants that have disproportion between the canopy foliage and root system.

If you are ready to choose an irrigation system, your options include drip irrigation with it's accessories like emmiters, micro spray heads and perforated tubing. Personally, I find drip systems to be a nuisance. The emmiters are hard to see, to determine if they are working. The moisture that they emit is hard to monitor. And the tubes get in the way of gardening tasks like planting and weeding and mulching. For these reasons, I prefer sprinklers with a properly designed irrigation system.

The drip irrigating may be required for exceptionally large yards with wells, or in arid climates where residents try to salvage every last teaspoon of water.

If sprinklers are used, and can run between 11 pm and 8 am in the morning, they can be nearly as water efficient as a drip system. Sprinklers can apply a more consistent coverage in many cases, because drip emitter water moves downward and outward in an inverted cone shape. Sprinkler water is an entire layer that moves downward.

Good soil preparation and mulching allow better water infiltration from irrigation. When soil is porous and not compacted, water can be applied quicker. This means that each irrigation zone will operate for less time. More irrigation zones can operate in the limited number of hours from 11pm to 8am (my preferred hours for irrigating).

Many homeowners - and landscape contractors - use drip emitters around trees that are planted. What happens when the trees increase their size and mass by 100 times in the next 20 years? Add more emitters? What if the the irrigation zone flow rate is maxed-out? This is why I don't care for drip emitters. Likewise, it's not wise to design an irrigation system with no room for an extra sprinkler on each zone.

One more thing about sprinklers: if the operating time for a zone of sprinklers is increased, the benefits and usefulness far exceed trying to increase the operating time for a zone of drip emitters. Provided that the soil is in good shape and that mulch is on the ground. Drip zones typically can operate longer in general. If the allotted time is stretched out even further, it "cuts-into" the time available for other drip zones to operate. Since sprinkler zones typically operate for a shorter duration, increasing the duration does not "rob" from time available to other zones as much. The larger the yard, the more important this is.