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We have about 50 meters (165') of cedar hedge between our row of townhouses, and the row behind us. Depending on the trees around it, it is between 10-12' in height, approximately.
I think they are white cedar, aka arborvitae; maybe a specie of Thuja or Leyland cypress.

You can see it on Google Maps here, and in those pictures.
We are in Montreal, Canada.

Our problem with those hedges is that they are getting more and more sparse; we see more and more through it, every year.

We had very different opinions on what we should do with them, to help them grow thicker. Should we cut their head, maybe to something like 8-9', and give them enough earth+compost+fertilizer? We were told by some people that those kind of trees should not have been left to grow this tall, but that if we cut their head too much, they're going to die. Others are saying it's fine to cut their head as much as 3-4', as long as it's done in the autumn or spring.

Ideas?

Thanks.

  • Need to ascertain which plant the hedge is comprised of, and the pics you've provided do not show plant detail - can't see the leaves properly, so can you add more useful pics for ID purposes please – Bamboo Jul 20 '17 at 16:55
  • @Bamboo This is the best I have with me right now: cl.ly/2a0v2N3v2W2d/o - Will add more tonight. Pretty sure this is cedar, but not sure which variety exactly (white maybe?) Thx. – Guillaume Boudreau Jul 20 '17 at 17:07
  • need a close up, clear pic of the sprays of leaves so its possible to see if there are overlapping scales in fours or threes and so on, preferably in a form that can be enlarged onmy device – Bamboo Jul 20 '17 at 17:40
  • which rules out Icloud pics - not possible to close in on those. Generally though, the only conifer you can cut back hard and have it cope is Leyland Cypress,commonly known as Leylandii, or Yew – Bamboo Jul 20 '17 at 17:47
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    Thanks. There are only three trees these could be,and none of them is Cedar - the three are Chamaecyparis, Cuppressocyparis (the aforementioned Leylandii) or Thuja - the cones could be any of these. As you're in the States, the likeliest is they're Thuja, since that's the most popular coniferous hedging tree over there. You can hard prune them, and most of the time, they will recover well - its just not 100% that they will, unlike Yew and Leylandii. Better done in June or early July where you are,but don't leave it later than this month – Bamboo Jul 20 '17 at 20:16
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One common problem is trying to grow them with the sides completely vertical - they do better if the top is narrower than the bottom. If the top is wider than the bottom it will shade the bottom out.

Your pictures seem to show an untrimmed (or untrimmed for a long time) "hedge" which I'd call more "a row of trees let go, than a hedge" and in that condition losing the bottom foliage is quite normal as the top of the tree shades it out. A dense hedge is an artificial growth mode forced by regular shaping/trimming.

  • @Ecnerwal...yay!! But this hedge is far better than I normally see with that angle. It is so important! You've been the only one EVER that has brought this up!! And don't deer just love to munch on Arborvitae? I've heard both that it is deer resistant and I have seen these plants just gobbled up by deer. – stormy Jul 20 '17 at 19:50
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Your hedge is indeed Arborvitae. Thuya occidentalis...some variety. One of the reasons your hedge is thinning might be spider mite. Need to get a loupe or magnifying glass to check this out.

Have you ever fertilized? Don't use compost that you don't know. Especially around a straight line of Arborvitae. Very susceptible to diseases...you need to give them a balanced fertilizer.

Whoever that has pruned these guys has done a good job. But they DO need to be topped. 8 to 9 feet is just fine! Who ever told you that was too high...? Topping them will take off the terminal buds that demand most of the energy. Taking those buds off redirects the energy into the rest of the plant.

Keep the sides at a slight angle...the top width has to be narrower than the bottom. That is very important. These plants are planted too close together but nothing you can do about that. Fertilizer, get some tree stakes and be done with it for a year.

Ideally, a hedge like this is made into two rows offset. This way if one dies it is not noticeable. 3' on center...

  • And I forgot to ask; do you have deer in your area? I've seen these hedges decimated by deer yet lots of information say that these plants are deer resistant. I don't think so! Check for insects, okay? – stormy Jul 20 '17 at 19:51
  • No deers here! We're in the middle of the city. – Guillaume Boudreau Jul 20 '17 at 21:22
  • Well we can cross that off the list. Do you have a magnifying glass? – stormy Jul 21 '17 at 0:13
  • I have a digital camera with a big enough zoom I'm sure would work. Where should I look? I Googled spider mite, and saw what it looks like. – Guillaume Boudreau Jul 21 '17 at 0:17
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    Look towards the center of one of your sparsest plants; look beneath the leaves and you might find webbing in the angles. You can take a piece of white paper and shake a branch onto the paper. Then you could use your camera? – stormy Jul 21 '17 at 0:26

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