I'm going to suggest starting your seeds March 15th to set out April 10th.....this is if you get quick germination and normal growth speed. I start mine about March 21st and set out about April 16th.

Could it be that the company was out of the seed when you orginally ordered the seed? When I place my seed orders, they always ask me if they are out ,if I want seeds shipped at a later date. If it will arrive too late for me, I say no. If thats the case, they would send the seed even if it is too late to plant in your area. I find it is usally, plants and seed potoaes, that are shipped at planting times for your area. People will order seeds mid-season for planting next year, so the seed company doesn't know when you intend to plant.

flowerfan is correct - PC iris are a bit fussy about root disturbance at the incorrect time of year. Fall is really ideal for dividing or transplanting of these. In general, fall division is recommended for spring/early summer blooming perennials and early spring for later blooming perennials. Having said that, I've divided Sibes at all different times of the year with good success, however they often miss a season of bloom if I did it at any time other than early fall.

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