One of my books says zone 7 and another says zone 10 for Acorus gramineus. 'Ogon,' the variegated cultivar, doesn't list any special hardiness. Maybe you could grow it in a container, and bring it inside for winter.

There are many varieties of sweet flag. The large variety (Acorus calamus) will be perfectly hardy in z4. The dwarf varieties are more variable. If you are just going to treat it as an annual, toss a little extra mulch over it in the fall...you may be surprised!

In my zone 5 garden all the small variegated Acorus do very well but, they look a little ratty after the winter. They are evergreen & take a bit of a beating from the winter sun and wind.
Once again I am advodating pots. Acorus looks great in a pot with your favorite perennial or annual. I have some dwarf green 'licorice' in a pot with an upright ivy ~ looks great and keeps really well in a cold unheated garage over the winter. It would also do well indoors under lights or in a sunny window for the winter.

Outdoors over the winter I would suggest a mulch of evergreen boughs rather than something like a leaf mulch. It tends to go a bit slimey if you cover it up too well.
A.

Thanks for the tips/info!

I think I will try planting it in the ground and will make sure I mulch it for the winter.

The more I looked at it after I got it home, I began to think it would make a nice container plant. But, this one will most likely go in the ground when I get the new area prepared. Geez, I looked at the temperature at 4pm and it was 52°---and, has been terribly windy last couple of days. Branches, twigs, and new shoots from trees all over the grounds around town. Not from storms---just wind. Arrrggghhh.

My Acorus gramineus 'Ogon' has sat in my pond for years. It grows on a very shallow shelf, between some largish rocks and the edge of the liner. Having read that it was not likely to be hardy in my Zone 5, the first year I removed it from the water and put it in the ground. Since then I have just left it in the water. The water freezes every winter, yet it does just fine.

It is in a fairly shady situation and does need cleaning up every spring.

Lost my first message so I'm trying again. I bought some variegated sweet flag, experiencing the same mishap of not knowing until I got it home that it is not hardy in my zone 4. I also learned of its need for moist soil so I created 3 bog planters, putting the grass in a pot with a mix of peat moss and clay and placing that pot inside another that is sealed at the bottom so it holds water. I put those pots in the ground and covered them with spaghnum moss. I'd like some advice on whether or not to bring them in over winter (Wisconsin) or just mulch them, leaving them in their pots. If I bring them in, what kind of care do they need? I didn't pay alot for them but I really like them and am afraid I won't be able to find replacements should they die over winter so I'm looking for the best survival technique! Ideas????

Dirty Thumb,
Why don't you just pot up one of your plants & bring it indoors. I think it would do quite well as a houseplant. Your indoor plant can be your insurance just in case your garden plants don't survive.
As I stated in a previous post (above), I do grow this plant in a pot as a patio plant. I also grow it in the ground in reasonably normal garden soil & in a 'manufactured bog'. I find it does equally well in all these situations. ie, it doesn't have to be in a really wet situation.
My potted plant (too large for the house) goes in our minimally heated garage (kept around 0C - 32F) and has overwintered this way for a number of years.