Sibs are easy to grow once you get them started, but starting them can be more challenging. If you plant them incorrectly, in the wrong place, at the wrong time of year, or give them the wrong kind of after-planting care, they may just sit there and rot instead of rooting in. It sounds like perhaps that is what happened to your Sibs.

Where did you acquire the Sibs? How did you handle them before you planted them? In what sort of soil did you plant them and did you fertilize them? What is your soil pH? How much sunlight were they getting? How deeply did you plant them? Did you mulch them? How much and how often did you water them? What were your temps like within a month after planting?

Laurie

You don't need all those questions. The amount of sunlight, pH (unless you are watering them with vinegar), depth and fertilizer are not likely to be factors in the cause of death of a sib from winter to spring, maybe over a few years. More than likely, the plants dried out, or they heaved a large amount over the winter and tore the roots off leaving the plants to rot. Siberians are not as tolerant of being chopped up as bearded iris. They suffer greatly from lack of water, especially new plantings. New plantings can sometimes be prone to rotting, if the roots are damaged. It does not suffer from rot like bearded iris do, it is more like a gang green sort of rot, where part of the plant dies, and starts to rot, and the rest follows. I make a note of planting siberians in the spring, so that they can get a firm root base before winter. They do not expell as much energy during blooming as beardeds and as of such, do not require much of a rest period before they can be moved. Keep the ground moist after they have been planted, until established. Mine typically take a year or so to adapt from planting, during which time, they grow slowely, and bloom more sparcely, but after that, they go crazy.
-note, they really don't like driveway salt.

Hmmm - I rather thought all of Lauries questions were relevant:
Where did you acquire the Sibs?
At a yard sale where they had been in a box dried out for several days? In a plastic package at walmart? (do they do that?) Dug or pulled roughly from a friends garden clump? All could spell disaster.

How did you handle them before you planted them?
Leaving them out of the ground for too long, or dividing them into too small sections will cause problems.

In what sort of soil did you plant them and did you fertilize them?

Very relevant. They prefer moist, friable soil, but not too wet. They are NOT "water iris", Too much wet will kill them.

What is your soil pH?
They prefer a slightly acid soil, but will tolerate some lime. I would think that they would be at there most susceptible right after planting, rather than several years down the track.

How much sunlight were they getting?
Maybe more relevant as far as bloom, but they won't grow in heavy shade.

How deeply did you plant them?
It is common belief that you should plant irises like Ducks sitting on the water. If this method was used for siberians they wouldn't last long. If they were planted too deep they probably wouldn't make it either.

Did you mulch them?
Mulch can shift and smother.

How much and how often did you water them?
Either too much or too little will cause problems.

What were your temps like within a month after planting?
here in Australia we plant Siberians in late Autumn. Our summers would do far more damage to new plants than our winters here. Once they are established, a clump will last reasonably well with minimal water through summer, but new divisions would most likely die. Typically plants good sized divisions planted in Autumn will have increased well by Summer, and may even flower here. They certainly should be showing some green by Spring, and if not, can be presumed dead.