The introduced riparian tree Russian-olive (Eleagnus angustifolia L.) is abundant throughout the interior western United States, except in the extreme south. Russian-olive seeds generally exhibit embryonic dormancy and require a period of cold stratification. We hypothesized that insufficient cold stratification limits the spread of Russian-olive to the south and conducted a laboratory experiment to test this hypothesis.  If the southern limit were determined by a failure to break embryonic dormancy, seeds receiving cold stratification representative of locations north of the southern limit would have greater germination success than seeds receiving cold stratification representative of locations south of the southern limit. We collected seeds from naturalized trees along the Rio Grande River near Socorro, NM, subjected them to varying numbers of stratification hours (hours between 0 and 5°C), and recorded date of first emergence. Treatments were applied in growth chambers and included: (1) two treatments within the range of stratification hours found naturally South of the southern range limit (Low) (544 and 605 hrs); (2) two treatments within the range of stratification hours found naturally North of the southern limit (Intermediate) (1486 and 1598 hrs); (3) two treatments with stratification hours greater than those found naturally (High) (2084 and 2435 hrs); and (4) a control (0 hrs).

Results/Conclusions:

The Intermediate treatments had the highest germination rates followed by the Low treatments, then the High treatments, and finally the Control. This pattern resulted in a quadratic relationship between stratification hours and germination rates (R²: 0.85; y =-3E-07x² + 0.0008x + 0.1807); germination increased with stratification hours until approximately 1540 hours, after which germination rates declined until they leveled off at approximately 2085 stratification hours.  Germination rates ranged from 0.19 to 0.78. In addition to having the highest germination rates, the Intermediate treatments had the lowest variance in emergence dates, indicating that the ideal range for germination was 1486-1598 hours. We expected germination to be high (greater than 0.50) in experimental treatments corresponding to regions in which Russian-olive occurs, and low in treatments corresponding to regions in which it does not occur. The Intermediate treatments, which correspond to a region where Russian-olive is prevalent, had high germination rates (0.61-0.78). However, the Low treatments, which correspond to a region where Russian-olive is absent, also had relatively high germination rates (0.51-0.54). We conclude that cold stratification requirements are not likely to be the sole factor limiting the southward spread of Russian-olive in the southwest United States.