In my experience (I'm in zone 6) most TB irises increase.

Dutch iris do not, and the Dutch iris peter out after a while. But they are lovely little things and are not expensive.

You can have both, why choose one over the other?

You can find Lots of TB pics at Snowpeak (hot link below) and to see Dutch iris, which grow from bulbs, do a google image search.

I have heard zone 6 is too cold for Dutch iris to naturalize as kprp says.

As for differences, Dutch iris take less space, are shorter and are wonderful cut flowers for mixed spring bouquets. TBs form much larger clumps, are much taller and aren't too great for bouquets. One stem is quite dramatic but make sure it isn't one that 'drips' after cutting.

Here is a link that might be useful: http://www.snowpeakiris.com/newweb/navpage.htm

I have bought Dutch irises twice that I can remember. They do not multiply and in fact dwindle, and this year what few bulbs are left only sent up foliage...the same as last year. I'll not buy any more of those for sure.

Sue

Don't want dwindlers. Thank you.

I am sorry someone else has had the same experience with dutch iris that I have had. I thought I had done something wrong with them.
Mine were gorgeous the first year and then nothing but foliage the 2nd year. it wouldn't be so bad if they would reproduce so that you would have flowers for more than 1 year.
Linda

I am going to try the Dutch Iris this fall. The experiences and success or failure with the Dutch Iris seem to be all over the place, nothing consistent. They are so beautiful that I simply have to try for myself. They don't cost much so really not much money ventured, simply time and effort to plant them.

My dutch iris were beautiful this year. Rita, they aactually bloomed before the tulips did. I hope to have another flush of them, but it may get too hot. The grower sent my order a second time when the Dutch iris were almost bloomed out, so I planted them "just in case" $15 for 40 was cheap enough to risk.

Oh, I love them. The dutch Iris were not something I even knew about until this spring. Someplace I was looking at iris (long before I bought any) had a picture of Dutch Iris and I thought to myself, I wonder what they are. I couldn't believe how beautiful they are.

Rita,
I love the too. of course, the early bloom is good too.
kay