The best time might depend on the type of propagation method and your location. The best method of propagation would depend on the habit of your thornless blackberries. In general, tip layering would be OK for semi-erect and trailing types; start this in late summer or early fall and then prune them off of the mother plant in spring, and transplant. Root cuttings can be done in fall, when the plant is turning dormant. Hardwood cuttings (and rooting hormone), when the plant is dormant; softwood cuttings, in spring, when the plant is beginning to grow.
Depending on your climate, the timing might change a bit. For best advice for your area, you might contact your county's Cooperative Extension. For contact info, see:
The best time might depend on the type of propagation method and your location. The best method of propagation would depend on the habit of your thornless blackberries. In general, tip layering would be OK for semi-erect and trailing types; start this in late summer or early fall and then prune them off of the mother plant in spring, and transplant. Root cuttings can be done in fall, when the plant is turning dormant. Hardwood cuttings (and rooting hormone), when the plant is dormant; softwood cuttings, in spring, when the plant is beginning to grow.
Depending on your climate, the timing might change a bit. For best advice for your area, you might contact your county's Cooperative Extension. For contact info, see:
http://www.almanac.com/content/cooperative-extension-services