Ask Mr. Smarty Plants

Sassafras Sassafras albidum (Sassafras) is a moderately fast growing, aromatic tree that is found in forest stands from southwestern Maine, south to northern Florida, and west as far as eastern Oklahoma and Texas. This description from “Silvics of North America, vol. 2 “ from the USDA has a very good chapter on Sassafras albidum, which addresses the question of allelopathy. It references the paper "The allelopathic influences of Sassafras albidum on old-field succession in Tennessee" Gant, R. E., and E. E. Clebsch. 1975. Ecology 56:604-615.  (click here to see abstract)

The following is a summary of the results of that study.

Vegetative surveys of 12 different stands of Sassafras albidum located in various study areas across the state were made. The vegetation was sampled to document which ground cover species occurred beneath and which were found outside the sassafras canopy. In general, more species were present and the percentage cover was higher outside the canopy.

Of the 37 species found in the survey, 17 were consistently higher in percent cover beneath or outside of the sassafras canopy. Ten of the 17 species that occurred outside  the canopy were as follows: 

annual herbs, Ambrosia artemisiifolia (Annual ragweed)and Cassia nictitans;

perennial grasses Andropogon virginicus (Broomsedge bluestem), and Tridens flavus var. flavus (Purpletop tridens);

biennial herbs Daucus carota, Rubus allegheniensis (Allegheny blackberry), and Rubus hispidus (Bristly dewberry);

perennial herbs Lespedeza striata, Plantago macrocarpa (Seashore plantain), and Solidago spp.

Seven perennials were more successful beneath the canopy; Asplenium platyneuron (Ebony spleenwort), Fragaria virginiana (Virginia strawberry), Lonicera japonica, Panicum virgatum (Switchgrass)., Parthenocissus quinquefolia (Virginia creeper), Paspalum setaceum (Thin paspalum), and Toxicodendron radicans ssp. radicans (Eastern poison ivy)  (also known as Rhus radicans).

Another part of the study tested the effect of some of the terpenoid compounds produced by sassafras on seed germination.  The following species  showed reduction in radicle growth in these tests.
Acer negundo (Ash-leaf maple), Acer saccharum (Sugar maple), Albizia julibrissin, Pinus virginiana (Virginia pine), Sorghum halepense, and Ulmus americana (American elm).