By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties

Often around this time of year, I’ll get a question from a small woodland owner asking whether it’s worth the trouble to try to water their newly planted tree seedlings. My standard reply has always been “No”.  Of course, in most cases, it’s not even a practical consideration, because the logistical challenges of delivering water to hundreds, if not thousands of seedlings on steep or rough terrain far from any water source far outweigh any potential benefits.  I also point out that our Douglas-fir trees are adapted to withstand dry summers. After all, millions of Douglas-fir trees are planted each year in Oregon, and most of them make it without any supplemental water. And, I know one or two woodland owners who have watered trees that they were concerned about, only to have them die anyway.

But this year, after fielding the question of watering young trees again, I started to think a little more about my standard answer. After all, all signs are pointing another drought year. Scientists predict that summers in the Pacific Northwest are only going to get hotter and drier in the future.  In light of these factors it doesn’t seem like an unreasonable question.

So, suppose you planted a reasonably small number of trees, and they are easily accessible (on flattish ground, near a road), and you have some method of getting water to the site (a water tank, a hose, etc.) These conditions could make watering feasible, so for the woodland owner who wants to give their baby trees the best start in life, or who just has a micromanager personality type, is it worth the time and effort?

I decided to try a small, highly non-scientific experiment to help shed light on this question, so I went out to a nearby clearcut that had been replanted last year. I brought a soil probe, a jug of water, and a couple of quart-sized containers (i.e. yogurt tubs). My intention was to see how different methods of watering affected the soil moisture profile.

The first thing to note are the weedy annuals surrounding the seedlings, a common condition two summers after planting. Watering in these conditions would defeat the purpose, as you would be watering the weeds too.

I took a soil sample with my probe in a bare spot near a seedling. I was able to get down about six inches. The first two to three inches were dry, but below that the soil was cool and moist. So by early July, the soil still had good moisture for the seedlings’ fine roots to access. A month from now, it may be a different story. [Update: I went back to site the last day of August and it WAS a different story: at least 4-6 inches of dry to nearly dry soil.]

Then I set out my two yogurt tubs in different bare spots, but in one, I punched a small hole in the bottom to allow the quart of water to seep out gradually. In the other, I just poured the water directly onto the ground. I thought that maybe delivering the water slowly would allow it to penetrate more deeply, whereas pouring the water might cause it to run off. I know this happens when I water my yard, and for landscape trees, it’s always recommended to water slowly, deeply and infrequently.

Two hours later, I probed the soil in the two watered locations. As expected, the entire soil profile that I was able to collect was moist. In the slow-watered spot, I brought up eight inches of evenly moistened soil. In the fast-watered spot, I could only get down about four inches, but there turned out to be a lot of buried bark in that spot, so that limited the probe. So I can’t really compare the difference between the two. Dang, I should have done a few more repetitions!

I’ll go back to my earlier point that Douglas-fir trees are adapted to dry summers. They produce fine roots mostly in the upper 8 inches of mineral soil, and any organic material on the soil surface acts as a sort of buffer or mulch. As the upper inches of soil dry out in the summer, I sort of wonder whether replenishing their moisture is beneficial to a new seedling or not. Perhaps that encourages more fine roots to persist in those upper few inches of soil, rather than developing more deeply where moisture remains longer into the season. Could that cause problems when you stop nurturing them? Take a look at the root profiles (below) of some three-year-old trees that were part of an experiment. From the left, the first and third trees received supplemental irrigation while the second and fourth did not. Note the differences in where the roots are!

Anyways, here is what I took away from this small investigation, for you to consider if you are thinking of watering your seedlings.

I stand my by earlier statement: No, watering probably isn’t worth the time and effort. But for those who aren’t totally convinced, or don’t have enough to keep them busy on their tree farm, I would say, you should only water IF you can answer NO to all of the following conditions.

• Are there weeds in the rooting zone of your seedlings? (If the answer is yes, you risk giving the weeds even more of a competitive edge by watering.)
• Is the soil moist 4-8 inches below the soil surface? You’ll need to dig some holes to find out, and you might be surprised.
• When you test out your proposed watering method (which you should), does the water run off before reaching the root zone of the seedling (roughly the top 8 inches of mineral soil)? You should try delivering water slowly and quickly and see what happens in your particular soil. The soil type where I tried this is a silty loam with moderate drainage according to the Web Soil Survey. If you have more clay in your soil, it may act quite differently.
• Were the trees planted more than two growing seasons ago? Any older than that, and their root systems are probably too large, and require too much water, for you to feasibly deliver.

Will we need to take measures to improve seedling survival in the hot, dry summers of the future? I think yes, but I don’t think supplemental water is the answer. We might go back to the practice of taking advantage of suitable “microclimates” when planting, such as planting seedlings on the north side of a stumps or downed logs. This is feasible at any scale and topography. In the long term, tree breeding programs will probably select for increased drought tolerance in seedlings.