Caring for Aged Perennials - Knowledgebase Question

Valley, NE
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Question by roqq
March 21, 1998
I have forty year old lilac bushes and iris plants that my grandfather planted. Each year they seem to flower less and less. Can you tell me what may be the problem, or what I could do to make them happier?

Answer from NGA
March 21, 1998
Are there trees surrounding the plantings that shade them? Many flowering plants require full sun in order to bloom. Regular pruning and feeding of your grandfather's lilac should restore it to its former glory. Iris multiply quickly and benefit fromdivision every 3-4 years. As you've found, if you let them go, they become crowded and don't bother to put energy into bloom. I think it's because they sense their success in propogating the species - there's less need to bloom to set seeds.

If the lilac has never been pruned, has lots of "suckers" sprouting from the roots, and looks weak or overgrown, prune it this summer after it finishes blooming. Remove all dead/weak/crowded branches, then remove 1/3 of the oldest branches. In the next two years, remove the remaining 2/3 of the old growth until the whole shrub is renewed. It probably could use some fertilizer - topdress around the base of the shrub with compost/aged manure.

Iris are best divided just after blooming has completed, when the plant enters a brief dormancy. It can also be done in early spring but this may delay flowering until the following year. Division is simple: I simply dig out the clump with a garden fork and pull/cut apart the rhizomes'll be surprised how many divisions you get! When replanting, enrich soil with compost, and keep the very tops visible, as this helps prevent root rot and insect infestation. Enjoy your rejuvenated garden!

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