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We have a 1m by 5m patch of land that is full of shale rocks about 5inches deep. Under it is some kind of tarpaulin/underlay to stop weeds.

We would very much like to plant some bee and butterfly friendly plants here such as lavender. I am getting conflicting advice online and from friends.

Should we remove all the shale and the underlay and fill it with topsoil/compost before planting or can we cover the shale with a few inches of compost and plant straight on top?

I am happy to do either, I would just like the best end result.

Also open to any suggestions on what to plant to help nature - we live in the UK.

Many thanks

Few plants will grow well if you have a waterproof impermeable barrier (i.e. your tarpaulin) only a few inches below the surface. They will either die in a dry spell because the roots can't go deep enough to get to any moisture, or they will drown in wet weather when the water can't drain below the barrier.

So I would certainly take the barrier out. What you do with the shale depends what "full of shale" really means and what you want to grow. For example lavender will prefer a well drained "rocky" soil rather than something "better quality" and more water-retentive. Even if you do add some topsoil, a surface layer of broken-up shale would help to keep the weeds down, and won't do any harm with any planting scheme based on perennial plants where you don't need to dig the area (and disturb the shale) regularly.

If it were me I would build up a layer of soil right on top of the existing bed. The shale will act as a good aggregate material to allow for adequate drainage. It sound like there is a weed suppressing barrier underneath it and so you are two thirds of the way to making a garden bed already. I am assuming the base layer is actually a weed suppressing fabric made to allow water to permeate. If it is blue or green you may actually have a tarp in place and I can not imagine anyone would use that to suppress weeds. You can test it by pouring water on top of it. If the water moves through the fabric fairly quickly you have weed suppression in place. If it just sits on top you have tarpaulin and in that case you should remove it to prevent standing water which can lead to root rot and mosquitoes.

If you need to remove tarpaulin I would reserve the shale from the top first. Next I would pull up the tarp and discard it. Replace the tarp with landscape fabric which you can find at a home repair store, a greenhouse or nursery, or online. It typically comes in rolls of 1m by 8m at minimum so you may have some left over. You will also need staples to keep it in place. These are sold separately and usually come in a pack of six. I would place them every 30 cm to 50 cm along the perimeter of the fabric.

Next replace the shale layer reserved be careful not to puncture the barrier you've put down. Finally add a 20 cm layer of top soil and another 15cm to 20 cm of compost to the top of that. This should provide you with a good base to plant milk-thistle or other flowers butterflies love. To find out how many bags of soil you'll need multiply your area 1m x 5m = 5m by the depth desired 5 x 20cm= 100cm. It is always better to have a little extra.

I hope this was helpful

Good luck!

Matt

Remove the fabric. There is a very interesting web site from Washington State University in the US called "Horticultural Myths, curated by Dr. Linda Chalker-Scott. Here's her take on landscape fabric:

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/landscape-fabric.pdf

In the US, landscapers almost always recommend fabric when doing new installations - mainly because they know that the fabric and the staples are cheap, the client pays for the mulch, and they can get an extra $2,000-$3,0000 to do a job that actually costs closer to $300. (yes, I used to work for such a landscaper). If you speak with anyone at a reputable nursery, however, they'll tell you to skip or remove any fabric.

If you want to read more myths debunked: https://puyallup.wsu.edu/lcs/