Depends...
Are they tropical?
If so, I would bring them out when night-time temps are consistently 50°F or better.
That's when I start leaving my peppers and various succulents outside.
In my area, this is sometime around May 1st.

Josh

I have tropical, and sub trop.... I would like to get my pomagranite, my elm, and my bursera simaruba"gumbo limbo," out side soon!!! i also have a few different ficus. and a brazilian rain tree. i might not bring out. The buds are sweeling on my harty bonsai that are already out. and i have leaf on my white birch.. I set my wisteria out. it has not been through dormancy yet. i started it from seed last fall. it seems to be OK!! so far. Just curious as to what others think..

My zone is 5/6 on the NE coast of MA.
More 6 than 5.
The only Tropical I have is a 5 year old cutting from a large Banyan tree in Disney World FL.
Watch the TV 7 day weather night time temps.
I have had it out doors for a month but bring it in if the temp goes below 38.(40 or 45 may be saver where you are.)
Paul