We have an old lilac tree whose base has split into about 6 trunks. We have cut back 2 of these at the end of May as they were at 45 degree angle to the rest of the trunks and at immediate danger of falling and splitting the base. We want to build a concrete base for a shed and need to build where these two remnants of trunks are. They are already sprouting. Should we cut them right back to the base at possible risk of damaging it, or is there another way to deal with them to keep the main trunks, and the tree, safe?
I would certainly consider removing this Lilac and it's roots before pouring the concrete. From experience, concrete is expensive, a new Lilac tree is quite inexpensive at the garden center!
Here is an article with more information.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/damage-from-lilac-roots.htm
Soon after they bloomed the year, the leaves are beginning to wilt, turn brown and fall off. It is happening branch to branch and slowly spreading through the entire tree.
Lilac can be attacked by some pests and disease.
This article will help you determine the issue.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm
Why are the leaves on some of the lilac bushes shriveled and yellow and the new growth is the same while other leaves and branches on the same bush appear normal?
Curling and yellowing leaves can indicate a pest issue or even a disease.
This article will help you pinpoint the issue.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm
My lilac bushes sprouted in the spring but the new growth is stunted, shriveled, yellow and never grew to normal size. Other branches on the bush are normal. Now this stunted growth is drying up in the heat.
Your Lilac could be suffering from Bacterial Blight.
Here is an article will help you.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm
Is it possible that new growth that forms dense clusters of yellow and stunted leaves is caused by phytoplasmas or is witches broom? Let me know what you think.
Sounds like a case of Bacterial Blight. This article may help you:
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm
Our lilac bushes are about 4 feet. Do we cut of the spent blossoms each year?
Yes you can snip the flowers each year to encourage more growth and energy into the plant. Further reading https://www.gardeningknowhow.com/ornamental/shrubs/lilac/pruning-lilac-bushes-when-to-trim-lilac-bushes.htm
How to promote flowering on otherwise healthy lilacs?
This article will help you.
https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-bush-not-blooming.htm