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Thuja Occidentalis trees often turn partially brown, dry and they never tend to recover on their own. My problem is definitely with entire branches (as opposed to individual leaves).

I water the trees with drip irrigation twice per day and additionally I water them from top top to bottom manually about bi-weekly, as my gardener told me that these trees should be 'in a constantly humid environment', e.g. watering as frequently as possible. I'm definitely not telling that the gardener was right (!), but I'd like to know whether there was any scientific experiment on these trees, examining what is the ideal environment for them.

Gardeners tend to oversimplify things, such as 'You have to spray neurodegenerative chemicals on the tree' and 'There is nothing you can do about it'.

I don't want to give up nor do I want to start carpet-bombing my garden with chemicals. Is there any scientific evidence:

  • What is causing the browning (drying) of various parts of the trees at random locations?
  • Whether removing (cutting) the dead leaves bring any benefits to the remaining parts of the tree?
  • Whether there is any non-checmical way to fight this?

Anyone tried various methods on Thujas?

Pictures attached.

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  • @Kevinsky provided a link in their answer to a similar question (gardening.stackexchange.com/questions/22098/…). It may contain the information you're looking for.
    – Jurp
    Aug 16 '20 at 14:24
  • @Jurp Thanks, but the link to the publication in the similar question is broken. I think that 'fall seasonal' effects can be ruled out in case of my trees because the timing is completely random (it's summer currently). Appreciate your effort though! Thanks. Aug 16 '20 at 19:49
  • FWIW, a small-ish branch on my mature arbor vitae turned brown in the past two weeks - I think it's just a seasonal thing with arbs. My young arbs generally come through winter just fine and then have a few brown spots in late spring, which I just prune out. In a couple of years the tree has covered those spots with new growth from nearby branches.
    – Jurp
    Aug 16 '20 at 22:50
  • The link isn't broken - perhaps you're not in the US? Here's the link on its own: mortonarb.org/trees-plants/tree-and-plant-advice/…. Note that the article says that, in times of drought, evergreens will lose needles or, in the case or arbs, branchlets. So - were you in a drought and, if so, did you water the arbs?
    – Jurp
    Aug 16 '20 at 22:53
  • @Jurp Are you sure that this link works for you? oakgov.com/msu/Documents/publications/… My problem is definitely with entire branches (as opposed to individual leaves). I water the trees with drip irrigation twice per day and additionally I water them from top top to bottom manually about bi-weekly. Do your brown branches mostly recover during winter or do they remain brown indefinitely? My trees are about 11 years old. Aug 17 '20 at 6:26
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@Kevinsky provided a link in their answer to a similar question that you may find helpful - the specific link is from the Morton Arboretum and discusses seasonal needle drop. It also says that, in times of drought, evergreens will lose needles or, in the case or arbs, branchlets., which is what your arbs are doing.

I think your issue may be with watering, especially if your regimen was interrupted for some reason (vacation, for example). Established arbs should need no more than 1 inch of water a week, preferably a one-time-per-week event (like a large storm).

I don't have any science results to give you, but I can cite a great resource: Michael Dirr's "Manual of Woody Landscape Plants" (5th edition). This is a common tectbook in university and college horticultural classes. Here's what he says about Thuja accidentalis:

Culture

... should be grown in areas of considerable atmospheric moisture as well as soil moisture; requires a deep, well-drained soil...

so far, matches what your gardener says. But - just a couple of dozen word later and in the same entry:

...in spite of the absolute admonitions about this species and cultivars, it will, once established, take considerable heat and drought; all over Cadillac Mountain, Maine, growing in rock crevices and exposed slopes.

As you can see, the text apparently contradicts itself. It doesn't, however: a young, newly planted arb needs a lot of water (and should be watered in well late in the year just before ground-freeze) in order to get established. Once established, though (after two summers in the ground, I'd say), then it doesn't need much if any additional water.

T. occidentalis is native east of the Mississippi and north of the Gulf states, so if you're in that area of the US (I'm assuming you're in the US), then you don't need to add any additional water. If you live west of the Mississippi, then you may need to supply additional water, but really, no more than an inch a week.

So, why the emphasis on watering? Because there isn't much else that could be causing the browning, except perhaps spider mites (which should be relatively obvious). The other reason for thinking it might be over-watering is that an extremely well-watered tree, such as your trees, will develop extraordinarily shallow roots; if the watering schedule is changed even a fairly little bit then the roots aren't deep enough to supply the tree with the water that's available.

If you want to change the watering schedule and amounts, I suggest you do so very slowly. Maybe cut off the overhead watering first (reduce to once a month, then eliminate), and then cut back on the irrigation a little at a time. I'd do this over a period of many months over perhaps two growing seasons. This will allow the roots to "sink" to a proper depth.

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  • I am in central EU, similar latitude like South Minnesota. Irigation is automated (daily) since this spring. Strange thing is that while we had a very dry June and July, the trees were holding up excellend; in August we've been having regular large rainstorms (at least 2x per week) and the branchlets turned dry (brown) right now when they have more than enough water :) Aug 19 '20 at 18:54
  • Southern Minnesota is within Thuja's range, with about 30" of rain a year. The branchlets would've actually begun dying earlier in the summer, if the death was caused by the dry spell. If you've just begun irrigation this year, then I would slowly cut it back this fall and then no longer irrigate next year (overhead watering during drought is fine, but about an hour at a time, once or twice a week to get the 1"/2.5cm per week amount).
    – Jurp
    Aug 19 '20 at 21:49

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