hello all .....getting my next outdoor bed ready for planting 8 new euphorbia: 4 Silver Swan and 4 Blackbird .....I will be top decorative dressing around them with lava rock but my question surrounds their roots / propagation: do these euphorbia grow larger by shooting off roots ie: spreading underground, then up ? or do they grow as "clumping" from a single plant base ie: shooting up new growth from the base only ? I am on Whidbey Island just N of Seattle in the Pac NW, as far as location goes ....thank you for the assist !