Do flowers which have bloomed, still require nourishment until the end of the season?

(Question)

The garden’s peonies and Irises have completed their flowering for the short season. Should I continue to feed them soil nourishment?
They’re going to get water, since I have soaker hoses set up. I used ProMix a couple of weeks ago in west end Toronto. There’s been plenty of water, and it appears there is a lot of hot weather coming this weekend.

I noticed some of irises came out of their roots, and I need to cut them, so the wind doesn’t help them get uprooted. We did just have a lot of wind and water over an extended period, and then it became very warm.

Any suggestions until winter?

(Answer)

Both peonies and irises benefit from fertilizing in spring, and again halfway through the growing season after blooming.  For both these perennials you should be using a low nitrogen fertilizer (such as 5-10-10), being careful not to apply it too close to the roots and stems of the plants as commercial fertilizers can damage stems and roots by burning them.   Nitrogen rich fertilizers (nitrogen is the first number in the series of three) can decrease or suppress flowering as they encourage leaf growth at the expense of blooms.  At this point you should also cut back the spent blooms on both plants.   Once your plants have been fertilized, remember to continue to water thoroughly.

If your iris rhizomes are protruding extensively from the ground, overcrowding may be the reason, and your iris may need to be divided: clumps should be divided every three to five years.  The best timing for this is four to six weeks after blooming, in this case, August.  In the fall, the iris foliage may be cut back to about 6 inches from the ground.   The American Iris Society’s website provides a clear step-by-step guide to growing and dividing iris, with very good photos:

https://www.irises.org/About_Irises/Cultural%20Information/Grow_Bearded.html

Two other informative websites are:

https://canadianpeonysociety.blogspot.ca/p/planting-instructions.html

https://www.cdn-iris.ca/

It is worth noting that some soil mixes contain slow release fertilizers as part of their formulation, and it can sometimes be difficult to determine the formulation, the strength,. and the release period, of the fertilizer component of the mix.   Organic material such as compost is always a good option for enriching and amending your soil.