Aptly named the Cobweb Sempervivum due to the web like filaments that are a feature, Sempervivum arachnoideum and its cultivars are one of the more fascinating species.
How to grow Sempervivum arachnoideum
This is a tough plant that survives dry conditions, in fact it requires dry periods to grow well.
Position
Grow this one in full sun to a little shade.
Soil/Potting Mix
A free draining succulent and cactus potting mix is ideal. Look for 30% grit or scoria with other ingredients including medium bark.
When using Sempervivums as a green roof you need to look at safety and weight bearing capacity before you start.
You do need to waterproof the roof first so as not to cause damage, so a proper waterproof lining installed correctly is a must.
Basically a tray lined shade cloth or similar and then a very light potting mix covered with a geofabric.
Slots are cut into the geofabric and them the sempervivums are planted in the slots.
It is best to consult with a roof plumber, builder or other expert before attempting to create any green roof.
Watering
If you are growing Sempervivum arachnoideum in the ground it will require very little extra water. You do need to ensure that drainage is excellent.
Growing Sempervivum arachnoideum in containers as with all Sempervivums requires a different watering regime, and it needs to be a wet/dry cycle.
The plants are watered and then the potting mix is allowed to dry before watering again. Frequency of watering will be perhaps every 2 weeks in summer and just a little every 6 weeks in winter when it is in dormancy. The actual frequency will depend on the climate zone and growing medium.
Just ensure that the soil is allowed to dry.
Do not mist spray, water the soil around the plant.
If the foliage begins to look limp and wrinkled or shrivelled you a re under watering.
Fertiliser
This is not a plant that needs much extra fertiliser. A half strength application of liquid fertiliser in spring should be adequate.
Temperature
Ideal daytime temperatures are around 50 – 80F (15 – 26C). Although this is plant that will tolerate frosts is will not tolerate cold and wet conditions.
