Best practice is to divide in early spring when new grown appears - they say waiting too long might result in a loss of blooms for the season. Blooms are also sacrificed if they've become overgrown, crowded and a "doughnut hole" appears... bare spot in the middle with a fringe of foliage on the outer edges. Since we don't have hot, dry summers, I tend to divide mine when they've finished blooming without any loss of bloom the following season.

They're tough, make sure you've got a sharp shovel.

They bloom when they bloom, unlikely that there's a way to extend the time in flower beyond, perhaps, different varieties. I have the royal navy blue Caesar's Brother and the white companion, Snow Queen. CB comes into bloom first.

I have found fall to be the best time. They take a while to recover.

Agree that fall is the best time, but it can be done almost any time, so long as you understand they may not bloom that year.

You may want to do a web search on "two fork dividing plants". It's one of those things that is easy to do, but hard to explain. Basically you put two digging forks back to back in the middle of the plant, and lever the two halves apart. It works extremely well with things like Siberian iris and daylilies.

what they said.. but lets simplify it a bit ... the plant itself doesnt matter.. there need not be different rules for different plants .. in this regard ...

the simple question is.. do you want flowers this year ...

if you do ... you sharpen your shovel.. and cut away half.. the half left.. will probably bloom ... with no insult to the root mass left ...

if you simply plot the other half in a new spot.. and treat it well.. it will also probably bloom .. but might not ...

if you divide the second half.. into a bazillion little pieces ... they might bloom next year ...

never forget.. flowering is sexual maturity ... and a babe has to grow a mature root mass ... to flower above ... so the smaller you go on dividing.. the longer it will take to clump and flower ...

you need the simple rule .. because.. often .. you have to do.. what you have to do.. when you have the time ... too many projects get left behind.. and never done.. because you think.. oh.. i will come back and do that in fall .. and then next spring.. you are standing there.. back at square one ... lol

ken

So very true, Ken.

As far as extending bloom, the particular plants you own can't be changed, but there are Siberian irises that bloom earlier or later and ones that have some rebloom, so you could extend the overall season of irises, though it would require shopping specialty nurseries. There are also Japanese irises that bloom a bit later than Siberians, but have similar style of blossoms.

I would, personally, split the irises shortly after they are done blooming. The need pampering afterwards, especially if summer heats up early. Keep them consistently watered until the roots re--establish, 4-6 weeks minimum. That way you should not sacrifice any blooms at all.

There is no way to extend bloom period--the flowers are on a timeline set by the plant's own genetic material from the second buds form until the last petal falls. As someone else mentioned, there are varieties that may bloom a little earlier or a little later. Pick up some of those varieties and plant nearby is your best way to keep blooming irises around for a little longer.

Thanks, everyone! I am going to try to divide them in a couple of weeks but it's such a huge job (I have about 16 large plants) I don't know if I'll get them all done. :)

kirsten reminds me ... the usual default ... again.. not plant specific ... is to divide after bloom ... when ever that may be ..

in other words.. enjoy the show.. then go after it ...

on some level.. in theory .. it should have time to regrow the root mass.. given you are a year out or so.. from the next bloom ...

if i had 16 plants... i might get rid of half of them ... lol ... offer them free to whomever wishes to come dig them out ... good bye every other one ...

ken

as with almost everything else do it when you feel like it and have the time.

There IS a well-established rule of thumb for when to divide perennials.......spring blooming perennials (like Siberian iris) are best divided in fall while summer or fall blooming perennials are best divided in spring. This is so the root development is maximized before the bloom period, therefore also maximizing the bloom potential.

However, reality often intrudes on theory and I'm with dave on this - do it when you have the time and inclination.

As to flower buds being formed in fall, that is simply just not the case with any herbaceous perennial that dies back to the root crown in winter.

I divided my old (~15 years) clumps of Siberian Iris and not only was it not easy, the divisions were just eh - not really very vigorous and spotty flowering. I hit an end-of-season clearance and replaced those divisions with new clumps in fall 2013, which grew beautifully last summer. So, frankly, for the amount of labor required and less than great results, I say just ditch them and buy new plants if they aren't looking good anymore.

Thanks, everyone. We're selling the house and moving in mid-May, so with everything else I have to do, I think that I will tell the new owners that they need to divide them in the fall. :) They're young and a lot more energetic that I am, and I do remember that this is a long process (I think that there are about 16 huge plants...)