Seeds of the narrow-endemic Solidago shortii and of the geographically-widespread S. altissima and S. nemoralis collected and buried in 1992 and 1993 were incubated in light and in darkness at 15/6°, 20/10°, 25/15°, 30/15° or 35/20°C following various periods of burial in soil in a non-temperature-controlled glasshouse. At maturity in November, seeds of the three species germinated to 0–1% in light at 15/6°C and to 10–77% at 20/10°, 25/15°, 30/15° and 35/20°C. Seeds exhumed each April from 1993 to 1996 and incubated in light at 15/6° and 20/10°C germinated to ≥83% and ≥90%, respectively, whereas those exhumed each September of 1993–96 germinated to ≤2%and ≤40%, respectively. At 25/15°, 30/15° and 35/20°C in light, seeds of S. altissima and S. shortii germinated to ≥52% and those of S. nemoralis to ≥19/, regardless of when they were exhumed. Timson's index, which integrates percentages, rates and times for onset of germination, was higher at all temperature regimes for seeds exhumed in April 1995 than for those exhumed in September 1995. Freshly-matured seeds of the three species germinated to 0–11% in darkness. Furthermore, regardless of when they were exhumed, seeds of S. altissima and S. nemoralis incubated in darkness germinated mostly to only 0–9% over the range of temperature regimes. In contrast, ≤88% and ≤6% of the seeds of S. shortii exhumed and incubated in darkness each April and September of 1993–96, respectively, germinated, ≤1% of them germinating while buried in soil. Thus, although buried seeds of all three species exhibited an annual conditional dormancy/non-dormancy cycle, only those of S. shortii exhibited cyclic changes in their germination response in darkness.

The effect of dry storage under ambient laboratory conditions on after-ripening and survivorship was tested on seeds of the geographically-widespread Solidago altissima and S. nemoralis and the narrow-endemic S. shortii. Freshly-matured seeds of S. altissima collected in 1991 and in 1992 germinated to low or moderate percentages in light at 15/6, 20/10 and 25/15°C and to high percentages at 30/15 and 35/20°C, whereas those of S. nemoralis and S. shortii germinated to low percentages over the range of temperature regimes. After 0.8–1.8 years of storage, 1991 seeds of S. altissima incubated in light germinated to high percentages at 25/15, 30/15 and 35/20°C, those of S. nemoralis did so at 30/15 and 35/20°C and those of S. shortii at 20/10, 25/15, 30/15 and 35/20°C; 1992 seeds of all three species germinated to high percentages at 20/10, 25/15, 30/15 and 35/20°C. Freshly-matured 1991 and 1992 seeds of the three species germinated to low percentages in darkness over the range of temperature regimes, and only seeds of S. shortii germinated to high percentages after 0.8–1.8 years of storage. Compared with cold stratification, dry storage was only moderately effective in breaking dormancy in these three species. The primary difference in after-ripening of seeds of the three species was that seeds of the narrow endemic germinated to higher percentages in darkness than those of its two geographically-widespread congeners. Survivorship curves for 1991 and 1992 seeds of S. altissima and S. nemoralis and for 1992 seeds of S. shortii were of Deevey Type I; the survivorship curve for 1991 seeds of S. shortii was closest to Type II. Longevity of 1991 seeds of S. altissima, 1992 seeds of S. nemoralis and 1991 and 1992 seeds of S. shortii was <4.0 years, whereas that of 1991 seeds of S. nemoralis was <2.3 years; 5% of 1992 seeds of S. altissima were viable after 4.0 years.

Seeds of the geographically-widespread Solidago altissima and S. nemoralis and the narrow-endemic S. shortii were buried in pots of soil and placed in a glasshouse without temperature control. After 0.3–4.3 years of burial, some seeds (21–60%) of all three species were viable and they germinated to 75–100% during 2 weeks of incubation in light at 30/15°C. Soil samples collected from several population sites of S. altissima, S. nemoralis and S. shortii were placed in the glasshouse and monitored for seedling emergence. During the first, second, third, fourth, fifth and sixth springs, the number of S. altissima seedlings m−2 emerging was 108–1080, 8–494, 0–520, 0–69, 0–6 and 3, respectively, of S. nemoralis was 108–1122, 17–667, 0–42, 0–6, 0, 0 and 0, respectively, and of S. shortii was 61–1753, 0–25, 0–6, 0, 0 and 0, respectively. More seedlings emerged from disturbed than from non-disturbed soil, but the differences were not significant. Thus, although some seeds of all three species buried in pots remained viable in soil throughout the 4.3-year burial period, longevity was greater and size of seed bank larger in field-collected soil samples containing seeds of the geographically-widespread species than in those containing seeds of the narrow endemic. Although 34 studies have reported seeds of 17 species of Solidago present in soil seed banks, the present study is the first to show, conclusively that Solidago can form a persistent seed bank.

Regardless of whether or not seeds of the geographically-widespread Solidago altissima and S. nemoralis were exposed to light in autumn, those ‘dispersed’ in autumn (15/°C) or winter (5°C) required 12 weeks of light in winter to germinate to ≥80% in darkness in spring (2 weeks at 20/10°C). On the other hand, seeds of the narrow-endemic S. shortii dispersed in autumn and exposed to ≥2 weeks of light in early winter germinated to ≥77% in darkness in spring, and those dispersed in winter and exposed to ≥6 weeks of light germinated to ≥82%. S. altissima and S. nemoralis seeds not exposed to light during any season germinated to only 0–1% in darkness in spring, whereas S. shortii seeds germinated to 45–56%. Seeds of S. altissima and S. nemoralis kept in darkness in autumn and winter needed a 1-day (14-h photoperiod) light exposure in spring to germinate to ≥75% in darkness, whereas those of S. shortii required only one 5-s exposure. Cold-stratified (nondormant) seeds of S. altissima, S. nemoralis and S. shortii exposed to light with a high far-red/red ratio germinated to significantly higher percentages than dark controls and freshly-matured and lab-stored seeds. Results of this study suggest that a soil seed bank of S. shortii should be smaller and be depleted at a faster rate than those of S. altissima and S. nemoralis, and portions of the seeds of the three species can germinate in the far-red-enriched light under plant canopies.

Solidago shortii is endemic to a small area in northcentral Kentucky (USA), whereas two of its sympatric congeners, S. altissima and S. nemoralis, are geographically widespread. Seeds (achenes) of S. shortii (0.370 mg) are significantly larger (PLSD, P=0.05) than those of S. altissima (0.070 mg) and S. nemoralis (0.068 mg). Germination percentages of freshly-matured seeds of the three Solidago species collected in November 1991, 1992 and 1994 were 0–2% in light at 15/6°C, 1–37% at 20/10°C, 9–56% at 25/15°C and 10–85% at 30/15 and 35/20°C. Stratification increased the percentage and rate of germination and decreased the time to the onset of germination (measured by Timson's index only at 20/10°C in light) in the three species. Following 12 weeks of cold stratification in light, seeds of the three species germinated to 72–100% in the light and to 22–100% in darkness over the range of thermoperiods; those cold-stratified in darkness germinated to 39–100% in light. Freshly-matured seeds of S. altissima and of S. nemoralis germinated to 0–4% in darkness, whereas those cold-stratified for 12 weeks in darkness germinated to 0–28% in darkness. On the other hand, freshly-matured and cold-stratified (in darkness) seeds of S. shortii germinated to 0–13 and 13–73%, respectively, in darkness. Under near-natural temperatures in a glasshouse without temperature control, germination of the three species peaked in March. Thus, the primary difference in dormancy-breaking and germination requirements of the three species is that the endemic germinates to a much higher percentage in darkness than its two congeners. Seeds of S. shortii do not have any special dormancy-breaking or germination requirements that could not be fulfilled outside its present-day geographic range.

Seeds of the narrow-endemic Solidago shortii and of the geographically-widespread S. altissima and S. nemoralis buried in December 1993 were exhumed in June 1995 and given 10 cycles of 1 day wet/5 days dry, 9 of 2/5, 8 of 3/5, 7 of 4/5 and 6 of 5/5 in light at 30/15°C; the control was kept continuously wet during the experiment. Seeds of the three species incubated on wet substrate for 3, 4 or 5 days germinated to ≥47% during the first cycle. On the other hand, seeds kept moist 2 days germinated to only 4–26% in the first cycle, and none kept moist for 1 day germinated. Cumulative germination percentages of seeds of all three species at the end of the final cycle of the 1/5 treatment were 0–4%. In the 2/5 treatment, cumulative germination percentages of S. altissima and S. shortii seeds at the end of the final cycle were 50 and 41%, respectively, but that of S. nemoralis was only 4%. For all three species, cumulative germination percentages were ≥55% at the end of the final cycle of the 3/5, 4/5 and 5/5 treatments. Control seeds of the three species germinated to 85–99% after 2 weeks, and no additional seeds germinated during the remainder of the experiment. High percentages of seeds were viable in the treatments and control at the end of the experiment; however, some seeds of S. nemoralis and S. shortii given 1/5, 2/5 and 3/5 treatments became dormant. The ecological implication of this study is that seeds of the three species will not germinate on the soil surface after brief rainfall events in summer. The germination response of the narrow endemic is similar to that of its two geographically-widespread congeners.

Germination responses of nonstratified (conditionally dormant) and of cold-stratified (nondormant) seeds of three Solidago species were tested in darkness at 30/15°C following various lengths of exposure [0 (control), 5 and 30 min, 1, 6 and 14 h] to green (mostly 500–600 nm) or white (400–700 nm) light. Prior to exposures, nonstratified seeds were imbibed in darkness at laboratory conditions for 24 h, and cold-stratified seeds were kept in darkness at 5°C on a moist substrate for 8 wk. Nonstratified seeds of S. altissima, S. nemoralisand S. shortii germinated to 0–46%, 0–25% and 36–99%, respectively, in darkness following 5 min to 14 h of green or white light, whereas cold-stratified seeds germinated to 53–98%, 31–85% and 82–100%, respectively. Thus, sensitivity of seeds to both green and white light increased as they passed from conditional dormancy to nondormancy. Moreover, sensitivity to green and white light was similar to the hierarchy of seed size. Green safe lights must be used with caution in interpreting the results of studies on light/dark responses of seeds.