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Our new home has several rhododendrons that look to have gone unmanaged for many many years. The largest, in the pictures below, has grown well over the roof.

When should this get pruned? How far down should it get pruned at a time? What's the ideal height for a plant like this?

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I'd start thinning your rhody now. Take out all the little branches right down to the main trunk. Any branches that grow towards the center of the shrub. Any branches that are unhealthy. Branches that are redundant. This fall, head your shrub back by cutting the tips down to a bud that faces the outside of the shrub. Make the height 10'. Use alcohol on your pruners to clean before and after.

The problem for your shrub is that it is right next to the concrete foundation. Rhododendrons love acid soil. Concrete raises the pH and makes some chemicals in the fertilizer unavailable. I'd get a soil test to find the actual pH and then add sulfur. Fertilize now (unless you've already done so) with a fertilizer made for acid loving plants. Don't fertilize again until spring.

You also have the Japanese Rhododendron beetle. You can see the little nibbles on the edges of the leaves. That isn't the problem. The problem is in the roots near the bottom of the trunk where these beetles lay their eggs. The larva eat at the roots and reduce the vigor of your plant. Because the pH is off your rhody is more susceptible to these common insects.

Adding an article on this pest: Japanese Beetle. Please don't use the chemical pesticides. It really is not necessary. I am assuming there is a lawn just out of the pictures you sent. I'd try to talk with the previous owners about what they have done in terms of fertilizer and pesticide. Rhodys this large make beautiful LARGE bonsai looking small trees. Also be aware that plants don't live forever. This guy looks like he's almost reached his limit. Another question to ask the former owners. Do they remember planting this shrub?

Keep the pruning sessions spread out. Don't remove more than a quarter of biomass every few months.

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    Just an aside, I can't help it, grins...your fence is rotting at the bottom. Remove all soil, mulch, grass, weeds so that there is 2" of air between the fence boards and soil. – stormy Aug 15 '17 at 18:30
  • oh yeah- the whole fence is a rotted mess. It's all got to come out. – JeffT Aug 15 '17 at 18:37
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    @JeffT Generally you should prefer to keep the bigger branches because they have more leaves (which the tree needs for energy) than smaller ones, but you can remove bigger branches if they're growing in a way that you don't like (e.g. crossing other branches, aiming for the house, you just think it makes the whole tree ugly, ...). Pruning can be a bit of an art because you have to think of how a cut might make the plant look as well as how it will respond and regrow. Luckily Rhododendrons are robust plants given the right conditions, so as long as you don't go crazy while pruning, you'll be OK – Niall C. Aug 15 '17 at 19:48
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    Little branches are little branches because they and their foliage are not supplying as much photosynthetic energy as the larger branches with foliage that gets the most light. I look at the diameter of branches. When you have a wimpy diameter branch attached to a far larger diameter branch that little branch should go. By pruning we are doing what the shrub or plant is going to do eventually. We are speeding up energy redistribution by artificially cutting off energy users that don't produce enough to support their own basic needs. Yes taking the little niggly branches to leave the large – stormy Aug 15 '17 at 20:44
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    ...trunks. Keeping a nice canopy of leaves to be able to produce the food the plant requires for maintenance. These shrubs in shade will not be very floriferous because making flowers takes lots of energy. A plant in big big trouble will all of a sudden put all its energy into making seed in order to allow it to survive through progeny. Use by pass NOT anvil pruners. Keep the main blade closest to the live plant. Sharp blades. Felco makes the best pruning tool IMO. The actually allow you to replace blades and keep the original tool forever. – stormy Aug 15 '17 at 20:49

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