Answer: If your lilac suddenly changed its blooming performance, think back to your pruning. Pruning is best done immediately after blooming and at no other time during the year because you will be removing flower buds for the next year. This is the most common reason for poor bloom on lilacs. Another reason might be that the wood is old and the shrub needs to be rejuvenated. Rejuvenating means removing one third of the oldest wood at the base for three years in a row. This might stimulate new vigorous growth with better blooming potential. Finally, you might take a very close look at it for pests such as borer damage in the trunks or stems -- a problem of this sort could be sapping the shrub's energy.
In terms of soil amendments, lilacs seem to appreciate a topdressing of well rotted manure and/or compost in the fall and/or early spring as well as some balanced granular fertilizer applied according to the label instructions in early spring. If you have more doubts about your soil, you might want to run some basic soil tests and add additional amendments based on the results.
Your rhododendron might have suffered from the drought and died back as a result, it might also be suffering an infection of some sort. If newly planted, it might not be rooting into the surrounding soil -- these plants require a humousy acid soil. Based on your description I can't make a definite diagnosis of the problem. You might wish to consult with your County Extension (350-2540) for a more specific identification of the problem and suggested controls. They can also help with soil testing and interpreting the results.
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