How soon after they bloom can I cut back irises and peonies?
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let the fries die down on there own, the bulbs feed fro the foliage,peonies just dead head the flower itself.
Fall is usually best time
generally 2 - 4 weeks is what I have read on gardening sites.
Both of these flowers use the leaves to absorb for food, so it is most likely best to wait until the leaves all look dead. here in Texas that is usually late summer.
peonies should not be cut back until fall unless it has mold or other disease. The seed pods left from the blooms is next years bloom food source. Iris should be cut back after leaves yellow and starts to droop, again it's the bulbs food source.
You can remove the blooms immediately, but the greens need to remain until they wither, turn brown or the end of the season. Since irises grow from a bulb, the only way the bulb receives nourishment involves leaving the leaves intact to take nutrients from the air and transfer to storage for the next growing cycle.
Need to let folage die back on irise, tubers need the folage to thrive.