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.