Sempervivum Hens & Chicks Soil Requirements
I am working on a new green roof technology for sloped roofs and I would like to use Sempervivum. I was wondering what the minimum soil depth requirements are? The application would be for climates like that in Ashland.
Thank you for checking in with Ask an Expert. We will make some assumptions before addressing your primary question of "soil depth for sempervivum" as well as offering you some alternatives to that plant genus. First of all, we assume you know what the structural strength of the roof is (or is to be) in order to calculate the load of soil that can be used. Secondly, you know the exposure (sun or shade) of the target roof.Third, you know what provisions will be (or do exist) for drainage, although sempervivum and the alternative sedum genus both need minimal water. Both of these plant genera have species which will grow well in USDA Zone 8 (hardiness factor to 15-20 degrees average of the lowest winter lows over time). Portions of Ashland (higher elevations) are in USDA Zone 7 (colder weather, less tolerant in terms of plant life). Most good nurseries will know what are the hardiness zone of a particular plant; be cautious of using the zones listed by Sunset Western Garden Book, as they are proprietary zones and code Ashland and the Rogue Valley as Sunset Zone 7 (a mix of rainfall, temperatures, exposures.)
Given all these items of concern in planning a roof garden, your soil for both plant genera needs to be lean (stingy with nutrient), gritty or pebbly, with sandy or silty minimal loamy "dirt". Both sempervivum and sedum have cultivars which will grow in a minimum of soil over a rocky substrate--i.e., some are great alpine plants.
We have given you a choice of genera because sempervivum tend to be higher maintenance. The central "hen" of those types will set seed after blooming and die out. This will leave a "hole" that the "chicks" need to fill over time, or else the maintenance of removing the dead plant and replanting a cutting or "chick" could be burdensome. Not all sempervivum or sedum survive full hot sun, another consideration for the Rogue Valley's hot summers with no rain. We strongly recommend that you consult some plant references for species of both sempervivum and sedum. You will find many varieties in color, height at maturity, width at maturity, and tolerance to sun, shade, frost, drought.The summer, 2018 issue of Pacific Horticultlure magazine has an excellent photo spread on sedums. The previously mentioned Sunset book has information on culture and tolerance of 3 sempervivum species and at least a dozen low-growing ground-cover type sedums. Enjoy planning your project while you investigate its many possibilities.