Most seeds germinate in fall after the first rains, but some seeds can germinate year round under favorable moisture and temperature conditions. Buried seed of Scotch and Illyrian thistle can remain viable in the soil seed bank for at least 7 years and possibly to 20 years or more. Yearly seed production and seed dormancy are highly variable depending on environmental conditions. Germination of newly matured achenes is also stimulated by the presence of nitrogen, cold stratification, and fluctuating temperatures. Chilling sensitizes recently matured achenes to photoperiod, with 8 hours of light being optimal for germination.
Achene burial induces dormancy.
Achenes recovered from soil are sensitive to light quality, but not photoperiod, and are less responsive to the germination stimulators listed above. Germination of dormant seed in the upper soil layer is stimulated by low intensity burning, but imbibed seeds appear less tolerant of heating by fire. One plant can produce an average of 20,000 to 40,000 seeds. Seedlings emerge from soil depths to 4.5 cm, with 0.5 cm being optimal. Seedlings and flowering stems are highly palatable to goats.