Shallow soil with concrete underneath!

wannabesmallerholder

I recently moved into a rented property where the garden has been quite neglected. There is an empty north-facing bed, which I had planned to plant up this spring with some things like hydrangeas, dogwood and something with berries for winter interest and then fill in with some colour in between. However, this week I’ve just started preparing it and discovered that the majority of the bed is actually a concrete base, with about 30cm of (rocky) soil plonked on top! It’s just the two ends of the bed that are spared the concrete, so I could put a hydrangea in at each end. 

What (if anything) will grow in shallow soil with concrete underneath?! I was wondering about plug plants of wild flowers and then sowing a wildflower annual mix? And/or what depth do ornamental grasses need?

I was also thinking perhaps I could bank up some of the soil towards the back and make an edge with logs or something to create a similar effect to a raised bed and fill with compost etc to improve the soil. I could then create deeper soil at the back - it would still have concrete underneath, but it starts to drop away at the back so there would be a bit more drainage there. Would that work and what could I grow in the raised area? Could the dogwood go in there?

any thoughts gratefully received!

Loxley

Oddly enough most of my garden is about 30cm of soil over various types of hardstanding, although mine is on a slope so there's no issue with drainage. For most plants it seems to be fine. (Perennials don't need much soil depth really, especially grasses).

If it's a small raised bed you might find it dries out easily though.

wannabesmallerholder

Ok thanks @Loxley underneath it is a solid concrete base, not broken up concrete (I’ve just edited my post to say that as maybe “hard standing” wasn’t the right description!) - do you think that would still be ok? 

Loxley

This is for green roofs and gives an idea of soil depths (it assumes that a very stony 'green roof substrate' composed of crushed brick and a bit of green waste will be used).

To be honest I don't think you should feel too constrained. I would avoid things that like a moist soil, as I do in my garden, although Hydrangeas have done OK for me (paniculata).

wannabesmallerholder

Thank you @Loxley - that is really helpful!

Loxley

My hardstanding is mostly tarmac as far as I can tell! 

Phillip55

an alternative is to build raised planting areas using rocks to build enclosed "dry stone wall" effect borders. No digging up concrete and an extra advantage is you can fill each separate bed up with whatever soil the plants you want like. Camelias, blueberries etc fill it with ericacious soil. Lime loving plants.....no problem. Bog plants? Fill it up with clay rich soil. Its quick and easy to do. Ok an initial outlay in rocks and soil but plain sailing forever after that. You can even line one with a pond liner and have a raised pond. Options are endless.

robairdmacraignil

Centranthus ruber and buddleia are found self seeded on old buildings and walls near where I live so they don't need much soil to do well.

debs64

Maybe treat the beds as really large pots? Lots of feeding and watering and I think many things will grow well. 

Stephanie newish gardener

robairdmacraignil said:

Centranthus ruber and buddleia are found self seeded on old buildings and walls near where I live so they don't need much soil to do well.

Centranthus (valerian) is a prolific self-seeder, but fortunately the seedlings tend to pull out quite easily if you catch them  when they are small.  It is otherwise a really good value plant, flowering for months, especially if you periodically dead-head it (helps stop the seeds developing too), and it attracts the hummingbird moth.

BUT.......for years I've periodically wondered who was letting their dog poo on or near our gravel parking area, as in certain conditions I can smell it at the front of the house where the centranthus is. We live out in the country and so are not really a hotspot for local dogs to make a toilet, and there are lots of badgers that build their latrine in the verge across the road, so I put it down to that.  And then I read recently in a gardening magazine that centranthus can smell like dog poo in damp conditions!!!! It does it to attract flies as pollinators apparently! 
That answered my question about the smell and confirmed I wasn't going mad, but you might want to be aware of that quirk, albeit only an occasional one!