Good to the last drop

As part of our discussion of the relative merits of fall planting, Linda mentioned an article in Arboriculture and Urban Forestry that suggests that frequent, light irrigation might be better for landscape trees then the usual recommendation of infrequent soakings.  While I will withhold final judgment until I see the article (I did a scan of the last two year’s table of contents for A&UF but missed the article in question), here’s my rational for following the standard recommendation.


First, the context.  In discussing landscape tree irrigation I am talking about watering trees during establishment, typically during the first year after planting and maybe the second if the tree is lucky.  The goal of watering in this case is ensuring survival.  The questions are whether deep soakings are more likely to encourage deeper rooting where water availability is less variable than near the surface after irrigation ceases and whether infrequent watering increases drought tolerance over more frequent irrigation.


Roots follow resources
As my Woody Plant Phys students quickly learn, we avoid the teleological ‘roots seek out water’; nevertheless, roots do proliferate where resources are available.  A couple of illustrations.  As a Tree Physiology Project Leader with International Paper I supervised a 25 acre hardwood fertigation trial.   Trees were watered daily via drip irrigation system with emitters spaced every 3’ down a row.  As part of the study we did periodic root harvests.  My technicians quickly learned it was an easy job: just look for the drip emitters – every three feet there was a mop of roots right next to the drippers.  The notion of roots following resources is also widely reported in the ecology literature on tree utilization of ‘patchy resources’ (e.g. Gloser et al. 2008 Tree Phys 28:37-44 ).  Other factors being equal deeper watering should result in deeper rooting.


Trees habituate to frequent irrigation
Another short rotation forestry example.  In eastern Washington and Oregon forestry companies Potlatch and Boise Cascade operated intensively managed ‘fiber farms’ which grew 70’ tall, 7” diameter hybrid poplars on a 7 year rotation.  To maintain these growth rates, trees were irrigated daily.  But there was a downside: If one day’s irrigation was missed the leaders to the trees would start to wilt.  Three days without water would result in leaf drop. The daily irrigation was great for growth but it turned the trees into physiological wusses.


Periodic water stress improves drought tolerance and survival
A common adaptation for trees to tolerate drought is osmotic adjustment, which is an active accumulation of solutes that enables plant cells to maintain turgor pressure during dehydration.  Plants that have acclimated to stress via osmotic adjustments and other physiological adjustments are able to survive better during prolonged drought than plants that have not been pre-conditioned.  For example ponderosa pine seedlings that had been subjected to brief drought events survived a terminal dry-down two weeks longer than seedlings that had been watered 3 times a week before the final dry-down (Cregg 1994 Tree Phys. 14:883-898.


What would it take to change my mind?
Obviously some of my examples here are anecdotal (though there’s plenty of hard data on osmotic adjustment and other drought conditioning effects on trees).  To recommend frequent (2 or 3 times a week), shallow irrigation I would need to see: a well designed and executed experiment that compared frequent irrigation to periodic (once every 7-10 days) applying the same amount of water weekly (0.5 to 1” per week) for the first year and then documented improved survival of the trees after irrigation had been discontinued.  I’m not saying it’s not possible but it goes against my personal observations with irrigated trees in a variety of settings and relevant data with which I’m familiar.

11 thoughts on “Good to the last drop”

  1. Bert, I’ll look at the May issue of AUF tomorrow at work (it’s not on line yet but I think that’s the one it’s in. If not, I’ll keep looking). Anyway, many nonagricultural areas have very shallow soils. Deep watering in these areas does little good except to create runoff. This can be compounded by soil compaction, which reduces soil oxygen. If there’s not enough oxygen, no amount of water will encourage root growth and in fact makes the problem worse by creating hypoxic conditions. Finally, the all-too-common perched water table created by layering of different soils on top of each other will likewise create problems. My point is that many urban landscapes do not have the lovely, deep, friable soils common at horticulture research institutions, including my own in the Puyallup river valley. There need to be more experiments conducted under less rural conditions to create good models for home landscape maintenance.

  2. With all of the rain in the Pacific NW this spring, I know that the lush new growth will not be sustained during the summer drought without way too much watering. So, I will be pruning what doth offend me, come fall.

  3. Bert, take a look at Bryan et al in AUF, 36(2):57-65. In the introduction, the authors discuss prevoius research demonstrating that trees establish better and have greater growth with frequent irrigation as opposed to infrequent. Let me know what you think.

  4. Linda:
    Thanks for the tip. The data from the Bryan study doesn’t really have much to say one way or the other about the irrigation frequency question though they cite a couple papers by Ed Gilman that are relevant (Gilman et al. 1998 JoA 24:1–9 and Gilman 2004 JoA 30:301-310). First off, I’ve always liked Gilman’s work. He designs interesting studies; incorporates physiological measures, his studies are typically well replicated, and keeps a foot on the nursery side and the landscape side. I also like that he’ll often publish experiment 1 and 2 in a single paper where a lot of folks would try to milk two papers out of the deal. There’s a lot going on in both of these papers and irrigation frequency was just one of several variables in each. Bottom line, the papers demonstrate that live oaks growing in sand in Florida grow faster when irrigated more often. In terms of the question of survival once irrigation ceases, the frequently irrigated (daily to 3x per week) trees in both studies had 100% survival. There was some mortality in the infrequently irrigated trees (I think 3 out of 120 in one study and 5 out of 210 in the other). In the 2004 paper irrigation frequency is confounded with amount: Frequently irrigated trees received 216 gal. during the season compared to 40 for the infrequent. (By the way this is not a criticism – the principle goal of the study was to compare amendments not irrigation). There’s no doubt frequent irrigation improves growth – that’s why nurseries water every day. Whether or not this is a good thing in the landscape is an open question. If irrigation can be supplied ad infinitum, it may not matter. In many urban and community forest applications, however, we rely on hand watering or TreeGators filled from water wagons and. can only afford to do it for the first year, maybe two. I also have concerns about homeowners killing with kindness once I recommend watering every day or every other day. No matter how hard they try, most homeowners will have a hard time overwatering if the only once a week or once every 10 days. The other way to look at is this; for just about any location in the country. if you got one inch of rain every week or week and half though the summer would you have any issues with establishing trees and shrubs?

  5. Now Bert, you grew up in Olympia. You know darn well that from mid-June to the end of September we get virtually NO rain out here. That’s true for most of the country west of the Rockies. I also imagine that in some areas of the south, the rate of evaporation may make it difficult to keep soil moist. So while it might be nice to have as much consistent rain as you suggest, it doesn’t happen in “just about any location in the country.” And given the weird weather we’ve had in the last several years – and the theory it might get weirder yet – I’m not sure relying on rainfall is necessarily the way to go. I still believe that maintaining fine roots (i.e. not yet lignified or suberized) requires consistent moisture. Rhizotron experiments support this – fine roots follow the water and nutrients. When those dry up (pun intended), so do the roots.

  6. Linda,
    You missed my point entirely. I’m saying IF you gave one good watering a week – equal to about 1″ of rain per week – for the first year, in most locations you should have few problems getting trees established – at least as far as mositure is concerned.

  7. You’re right, I did! But an inch of rain is often delivered more slowly than many people hand water. An increased rate of water delivery means more can run off and/or move through the root zone before it’s used. The runoff especially true with bare soils that dry out to the point of becomeing hydrophobic between waterings.

  8. Linda, first let me say very interesting post, but my real question is if you have an update on this project? I would be interested in checking out the results.

  9. Seo, I haven’t been able to find any further research on this yet. When there is some, you can be sure that either I or Bert will post something.

  10. There’s a problem…seedlings DO need more frequent water because their roots are shallower, so I’d expect much higher early mortality from the less-frequently watered routine. And in pots, they need more water. I really think of the first year of any plant in the ground as a “weaning” from pot-watering levels to reasonable, sustainable levels of once every 5-7 days.

  11. Carl E. Whitcomb pointed out years ago that because most roots grow where the air is predominantly shallow root systems are the norm (except for certain desert adapted species). So that the repeatedly advocated soak-deeply-and-then-let-dry-out-in-alternation watering regime will therefore produce an adverse situation for plantings subjected to it.

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