Ask Mr. Smarty Plants

Butterfly Weed, Asclepias tuberosa is native to the eastern 2/3 of North America and its range extends well north of you into eastern Canada. While cold-hardiness is not normally an issue with this plant, the winter could have played a part in your plants' demise. If they were not well enough established when winter arrived, they could have been killed outright from that. Likewise, if they were in a spot where they were particularly well-protected through the fall and continued to make tender vegetative growth until the first freeze, they could also have died in that way.

Slugs can be an issue for young plants. If you otherwise have problems with slugs in your garden, that might be the cause.

The most likely cause is a fungal root or stem rot which attacked all of your plants. Newly transplanted plants are particularly susceptible to rot disease as there is often a lot of injury to the roots which are places for the disease pathogen to get into the plants. Also, new transplants are just weaker and more susceptible to fungal attack.

It is best to start Butterfly Weed early in the growing season and pinch any flower buds during the first year to encourage as much vegetative growth as possible. Since you have seedlings coming along now, if you have a way to hold them until spring in a protected area, do so. Otherwise, try to get as much growth as you can in the garden now, harden them as much as possible during late fall and mulch them before the first hard freeze. Watch for slugs or other causes of problems the following spring.