PlantFiles: Spineless Caper
Capparis spinosa

Seems nearly impossible from seed, which is the only way I've seen to get a plant (perhaps I need to look harder). Perhaps gibberellic acid will work. Cold-stratification regimens I have tried have been unsuccessful.

Propagation
Plants are grown from seed and by vegetative cuttings.
From seed:
Caper seeds are miniscule and are slow to nurture into transplantable seedlings. Fresh caper seeds germinate readily - but only in low percentages. Dried seeds become dormant and are notably difficult to germinate and therefore require extra measures to grow. Dried seeds should be initially immersed in warm water (40°C or 105°F ) and then let soak for 1 day. Seeds should be wrapped in a moist cloth, placed in a sealed glass jar and kept in the refrigerator for 2 - 3 months. After refrigeration, soak the seeds again in warm water overnight. Plant the seeds about 1 cm deep in a loose well drained soil media. Young caper plants can be grown in a greenhouse (preferable minimum temperature of 10°C or 50°F).

Stem Cuttings:
Collect cuttings in February, March or April. Use stems from the basal portions, greater than 1 cm diameter and 8 cm in length with 6-10 buds. Use a loose well drained media with bottom heat. A dip in a IBA solution of 1.5 to 3.0 ppm is recommended (15 seconds). A 70% rooting percentage would be considered good.