Last year I bought one that said the same thing on the tag.After looking threw some of my gardening books,I determined that it was not winter hardy at all.So brought in house for the winter it did great flowering on and off between waterings.It was hard to keep it watered,partly due to lack of TLC.
I set it on the porch in april,then we got that cold snap. I put it in a unheated shed because I did'nt think it would get cold enough to freeze.Well I managed to kill it three weeks before it would have been clear of frosts.
My opinion:frost kills Hibiscus

Yeah, when my buddy's wife indicated she wanted a hibiscus for a landscape plant I was thinking it wouldn't work. I was surprised the tag said zone 5.

I suspect it will do great for the summer and then it's a goner, but we shall see I suppose.

I have 3 hardy hibiscus going on their 3rd year. One of them my wife bought and planted in a pot and the end of the year I just dumped the pots out in the compost pile and the next summer there it was shoots coming up all nice. I have since transplanted in one of my beds and it has been doing great now for 3 years. These hardy ones end up really woody by fall and really are hardy. lol I could not even kill it when I tried.

If you are talking about altheas, we,ve got 2 of 3 that have survived for several years. They leaf out very late in Spring so give them time. Despite the fact that these have survived, I don't think they are anything special & likely would not plant again. They do look nice for the the week or so they bloom but the other 50 weeks they are............

My hardy to zone 5 dinner plate did not make it through the winter. It was in full sun, in a mulched bed. Very disappointed. I paid $12.00 for it at Steins. I wish the plant lables were more truthful. Judy

I've had three. One, a Rose Mallow, good to zone 5 but had only survived one winter and never came back. The one year it came back didn't show signs of life until June.

One Rose of Sharon, in southern exposure against the house, has turned into a sizable bush and growing bigger every year for 6 years now.
One other Rose of Sharon trained and purchased as a topiary, lasted 5 seasons and it looks like it died this winter. Services will be held at Waukesha dump Saturday.

I have 8 (now 9) of them. They are VERY slow to come up, all of mine but one are showing signs of life. But when I dug down a litte I found a small green shoot coming up. My favorite ones are Plum Crazy and Lady Baltimore. This year I also found Fantasia at the master gardeners sale in Green Bay. I love then for fall color at the back of a garden, but they do have a very short blooming period.

I grew them for about 10 years at my old house. An ideal site tho, so dont know if that is typical. I had them on a southfacing exposure against a 6 ft stone retaining wall.
They never ever showed their noses before June. Very late to break dormacy. Read tags carefully when buying, cuz some are intended to be pot plants for northern gardens, while others are perfectly fine in our zone 5 outdoor beds.
Pondy

I'm in zone4 and have 6 coming back from last year. We have 5 of them planted in front of a thick patch of Lilacs facing south.They were very slow to start, and I'd nearly given up, but they're now about 5 inches tall.

The 6th one is a surprise. It had gotten knocked over in a storm last yr before being taken out of the pot, so we set it aside assuming it was a goner. It got set alongside the potting shed and forgotten about. On Saturday we rediscovered the pot and in it was a lovely LIVE and growing hibiscus coming out sideways along the top of the soil reaching for the sun.

I set it in the sun, and it's already straightened out nicely. Mine are all "Luna Blush" from Menards.