I was sure waiting to see if someone else had an idea on this one.

My first thought is that maybe they are crossing with each other or maybe the hollyhock you had was a hybrid and the seed were not true to begin with. If several hollyhocks are close together they will tend to cross and the seeds will be totally different than the parent plant.

As far as the life of a single plant that I am unsure about, I do know that a neighbor of mine has had a hollyhock in her garden at least 7 or 8 years could be longer. I obtained seed from it and grew my own and I have had it for 2 or 3 years.

I'm not sure if the double hollyhock seed are hybrid varieties (the seed package just said 'double hollyhock mixture of colors')but I am quite sure the one's I grew did not cross with any single varieties. I guess then the seed then must revert back to the single varieties. I too have had the same plant come back again for a couple of years & some plant stalks in bloom way over 8-10 feet tall. but wasn't sure if they were true perennials or not. On the farm they just did as they wanted & we never paid much attention to them ... oh, as little girls we made flower dolls out of the open flowers & the un-opened bud ... can't remember how we did it now though!! :)

The doubles are so beautiful so was hoping they would re-seed & grow the same double shape!

I had some beautiful peach/ coral colour double headed hollyhocks. When they came back the next year, they were single heads. So gutted!

That's sad but good to know. I purchased a little packet of double hollyhock seeds but they all came up as single except for one.