Perhaps the most asked about plant in our garden right now is this purple leaved silk tree or mimosa, with it’s stunning chocolate purple fernlike foliage. The followup question is often, “but is it hardy” to which we answer, “yes, but…”. (Recommended hardiness zones are zones (6b) 7-10, and our garden is definitely zone 6a.)
This is the scoop based on our experience. Albizia julibrissin ‘Summer Chocolate’ is a fast growing ornamental tree or large shrub, introduced to the US market roughly 10 years ago. True, it is less hardy than the more common green leaved forms. In 2005, our first attempt to establish this tree failed. We didn’t site it well. We planted ‘Summer Chocolate’ in a spot where we thought it would look outstanding, but alas, it was the lowest spot on our property and the soil remains too wet there over the winter. Initially we were resigned to growing it as a container subject, but were reminded by the old adage: if a plant doesn’t grow well in one spot, try another, and then another.
Good drainage usually is key in wintering over borderline hardy plants. We planted the next specimen in well drained soil in front of a south facing stone wall, and are now enjoying this beauty for the 3rd year in a row. It did get some die back after the winter of 2009-2010, but broke growth along the lower trunk and quickly grew to 6′ that year. We won’t expect it to reach full height or width, and don’t really care. Tropical appearing chocolate colored foliage with a shrublike habit suits us just fine.
Those of you in milder winter areas (zones 7-10) can expect this tree to reach 20′ in stature and spread, with silky pink puffs of flowers in summer. Gardeners in zones colder than 6 might consider growing ‘Summer Chocolate’ as a stunning container specimen. The plant, pot and all, can easily be moved into an unheated garage or other protected spot once it goes dormant in late autumn.