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Hardening off trees with reduced irrigation

Posted by Cameron Denning 
Hardening off trees with reduced irrigation
October 21, 2015 08:07AM
I have been told that it is best to reduce or completely stop irrigating apple trees in september/october to help the trees harden off for winter. However, we have had an unusually dry summer and fall this year, and our trees went through a good deal of drought stress. Our trees are on dwarfing rootstock and drip irrigation. We still have not had a good rain to saturate the soils. I would like to water but am afraid of encouraging bud break of terminal buds. However, i also don't want the trees to endure any more drought stress than they already have. If irrigating late in the season re invogorates bud growth, wouldn't fall rain have the same effect?

Zone 8 Chimacum Washington. Peat soil high in nitrogen. M9 rootstock and trellised at 6 foot spacings. Varieties are all for hard cider.

Edited 1 time(s). Last edit at 10/21/2015 08:26AM by Cameron Denning.
Re: Hardening off trees with reduced irrigation
October 21, 2015 01:54PM
Do not allow the trees to go into winter with dry soil. The soils shouldn't be saturated either. Very dry conditions allow soils to freeze much deeper and in a more severe manner than moist soils. The roots and all underground portions of the tree (trunk, rootshank, etc) can freeze and damage much easier (and more severely) that they will with adequate moisture levels. The trick is to apply water slowly to bring soil moisture levels up and/or apply water after the trees are dormant/hardened off but before the soil freezes. The concern is if water is applied too early while the trees are still actively growing or during the early phases of the hardening off process. The water can solubilize nutrients in the soil making it available to the roots to take up, but only if the temperatures are high enough. Almost all physiological growth is tied to temperatures and without adequate temps, they'll stay dormant regardless. The other concern can be if there are nutrients in the water, but that is rarely an issue unless you have used your irrigation system to fertigate or you live in an area where soil/water sources contain high levels of nutrients, like nitrogen.

[/u] You live in an area where the winters are moderate and never too cold - CLIMATE DATA - so the bigger concern is making sure you can get the chilling hours needed so they break dormancy normally (don't prolong the hardening off period). As well, the peat soils suggest high water holding capacity (you already mention high N), so even though I don't know the specific soil moisture details, I suspect there is more moisture in the soil than realized (please correct me if I am wrong). I might delay the watering until the trees are hardened off and you're sure there aren't any prolonged, warm (>50F) periods on the way. As well, your soils likely don't ever freeze completely so the likelihood of damage from freezing soils/roots is less. So, the practical answer is: don't worry. Water the trees, but don't overwater. Delay based on weather forecast and how hardened off the trees are. But don't let them go through winter with super dry soils.

Mike Biltonen, Know Your Roots
Zone 5b in New York
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