Ask Mr. Smarty Plants

Delena Tull in Edible and Useful Plants of Texas and the Southwest , pp. 298-301, has a thorough discussion of "Irritant Dermatitis".

There are several plants that occur in the Central Texas area that can cause dermatitis of some sort.  First, there are the Tragia spp. that have 4 representatives in the area:  Tragia betonicifolia (betonyleaf noseburn), Tragia brevispica (shortspike noseburn), Tragia nigricans (dark noseburn) and Tragia ramosa (branched noseburn) and there are also stinging nettles such as Urtica chamaedryoides (heartleaf nettle) and Cnidoscolus texanus (Texas bullnettle). However, these plants would have caused an immediate stinging sensation that your daughter would have told you about.  Having been "stung" by all of the above, we can tell you they are unpleasant.

There are several other plant families that have irritating sap.  The Family Ranunculaceae (buttercup family) has several members with irritating sap.  The clematis vines are in that family and are named by Tull as having "highly caustic, acrid juices".  The following are the clematis species that occur in Central Texas:  Clematis drummondii (Drummond's clematis), Clematis pitcheri (bluebill), and Clematis texensis (scarlet leather flower). Euphorbia bicolor (snow on the prairie) and Euphorbia marginata (snow on the mountain), both in the Family Euphorbiaceae (Spurge Family) are two other plants that are common in Central Texas that Tull says have irritating sap that can blister the skin. Please see Tull's book for a discussion for other plant irritants that might not be as commonly encountered in our area.

For an explanation of the blisters appearing on your daughter's fingers, Mr. Smarty Plants has thought of several other possibilities:

1.  Perhaps there was a small Toxicodendron radicans (eastern poison ivy) growing in or near the verbenas that you didn't see that your daughter happened to touch. Or, perhaps some small animal walked through a patch of poison ivy and got the oil on its fur and then walked through the verbenas before your daughter picked them—a rather remote chance, we realize, but not completely out of the realm of possibility.  Many people have gotten poison ivy from petting their dog after it made a foray through the poison ivy!

2.  Stings from from fire ants (Solenopsis spp.) can cause blisters; but, again, your daughter would have noticed being stung by fire ants and let you know about it.

3.  Other insects, such as blister beetles (Family Meloidae), can cause dermatitis and blisters.