Since no one else is offering.... the only time I've had sedums self-sow in the garden was on completely bare, fairly hard-packed soil, in somewhat shaded locations but I think that was more because the shade kept the soil damper, rather than the sedums wanting to grow there. I don't think the straw mat would be the best idea, just too coarse, but some kind of cover in general would be helpful. Maybe a floating row cover?

If I were doing it I might try a thin mulch of fresh grass clippings (about 1/2 inch and hopefully weed seed and herbicide free) and then the seeds mixed with sand on top. Once watered, the seed will settle into the clippings and hopefully by the time the sedum sprouts the clipping will be just a thin layer of mulch they can grow through.

I wouldn't worry about late frosts, sedum are quite hardy and I've never seen late freezes do any harm.

Wow! Thank you! I found this information very helpful and I will definitely try your suggestions.

Thank you again.

I have done this, with various green roof mixes, simply by sowing directly into a gritty loam and perlite mix, topped with 1cm of flint based chick grit. The grit (make sure it isn't oyster shell) is fine enough for the seeds to settle, get enough light to germinate and also maintains some moisture. Once the little seedlings are up and growing, you can add another coupla centimetres of horticultural grit (although keeping the overall weight down is also an issue). I grow most of my seedlings in small pots but with a similar substrate and topping.

Caveat - some of the green roof mixes are less varied than I would have liked, so you can add extra seeds such as dianthus, talinums, other succulents and even fine grasses such as festuca.

Most of the initial preparation was building the frames, lining with waterproof , membrane, drainage guttering and a good friable, lightweight substrate. I like a drainage slope so I made a frame which divided into subsections of around 30cm x 60cm, in order to maintain level stability. It is a bit of a faff...but apart from gentle hand weeding, not very labour intensive once up and growing. Also, worth considering depth. Sedums really only need 3-4 inches of soil but green roofs can utilise as much as 45-60cm depth (for increased diversity) (not that I have ever done anything other than fairly small scale shed roofs bike shelters, bin stores and such, with a fairly limited palette of plants).

Oh wow, thank you for the tip! I think I’ll definitely add in some of the suggestions, the horticultural grit is great.

Appreciated!