Removing the tree and then using a raised bed with landscape fabric and a layer of gavel under it will solve the problem. The poisoning comes from the walnut roots as well as the leaves, etc. If a root cuts through the roots of the lilac, the problem will eventually show up. I had a walnut start growing near a rhododendron bed. It started killing off the rhododendron branches before I saw it. I killed the walnut by girdling it and dug down around the rhododendron beds with a grubbing hoe and removed all roots that were coming into the rhododendron bed. I was able to save the bed. The walnut took about 5 seasons to die, but it was depleting its roots while doing this. Steve Henning

The rule of thumb is that you can cut about 1/3 of the leaf area off at one time. In actual practice some people cut them back to a stump. It the plant is healthy, it will usually come back in either case. If it is in too much shade, it will come back a little spindly anyway. They need some sun to keep their shape and bloom. Prune immediately after the blooms drop to give it the most time to grow and form next years flower buds.

No need to pinch now, but would be helpful after bloom so no energy goes into seed formation. The downward leaves are a common characteristic of many varieties, as are yellow green leaves. Do check to make sure the rootball has not dried out. If it has, dig up and soak or water with a slight trickle at the plant base and recheck.

The leaves should look limp in the heat of the day. That is a normal response. However if they are limp in the morning, then that indicates that there is a problem. Make sure that the roots are getting wet. Rhododendron guru Harold Greer noted: "Quite often a plant will get completely dry and then no matter how much water you apply, the rootball will just keep shedding it. The top of the soil may seem wet, and the soil around the plant may even be very wet, but the actual rootball of the plant is bone dry. This is especially true for newly planted rhododendrons, and it is the major reason for failure, or at least less than great success with that new plant. It is hard to believe that a plant can be within mere inches of a sprinkler that has been running for hours and still be dry, yet it can be SO TRUE!". If soaking the root ball for about 30 minutes doesn't bring the plant back, then it has root rot from too much water and nothing will bring it back. The symptoms are the same for no water and too much water. The difference is that with no water you can water it and it will recover. With too much water, it gets diseased and will never come back. Before planting always test the drainage. This involves digging a hole about twice as deep as the height of the pot. Then fill this hole with water and see how long it takes to drain. If it doesn't drain in one hour, you have a drainage problem. The easiest way to solve such a problem is to use a raised bed with good well drained acidic soil.