Well that makes it more difficult to investigate, doesn't it? Other plants known for causing itchy contact dermatitis from either casual contact or from sap, Euphorbia (spurges and others,) Solanum (nightshade,) Juniper, Ipomoea (sweet potato vine, morning glory,) Sonchus (a dandelion plant with latex sap,) and others that don't come to mind at the moment.
At first I dismissed your mention of roses, but see page 2:
http://www.medscape.com/viewar...
"Even barely visible irritant fibers located on a plant can be problematic, as are those on rose hips (family Rosaceae)..."
Here are some others that may or may not be on your property:
http://plantcentral.wordpress....
This site has some great info about PI and a map that shows PI does grow in ID, though it's definitely possible that there's none on your property/in the immediate area. Patrolling for sprouts often can help keep it that way.
http://www.zanfel.com/help/ras...
Maybe something here would apply:
http://www.redorbit.com/news/h...
This site has such an extensive list, some people might need a hazmat suit to garden!
http://www.ct.gov/caes/cwp/vie...
You and he may have already arrived at this point in your thinking, but identifying the culprit may be a moot point though, knowing that something is giving DH a rash, he should take precautions to avoid exposure while doing similar tasks in the future. Long pants, socks, sleeves, gloves, be sure to not touch face or other unexposed areas while working, then immediately to the shower, depositing clothing directly into washing machine on the way to bathroom. Lately I'm sensitive to so many plants, my standard mode of operation is to no longer touch the sap of any plants, and I'm always washing my hands after touching plants. It's frustrating and more time-consuming, but I haven't gotten another rash since implementing these measures. Much better than so much scratching, and in some places on my wrist I have scars, well worth any measures to avoid.
