Starting from an unsightly pile of broken concrete and paving slabs, we decided to make a rock garden. This also allows for the garden to be raised allowing us to plant bulbs, which otherwise often rot over winter in the wet ground here.
Get the plants ready for your garden in advance. For this garden we wanted to have some bulbs, and also some lavender on top, so the bulbs were buried in loose potting compost around 30-50cm deep to encourage them to sprout roots and shoots, and cuttings of lavender were potted up in soil and allowed to sprout roots.
Laying weed mat underneath over the soil base before piling the concrete slabs on top can help to prevent weeds coming up around the edges, but if using weed mat make sure to cut holes under where you are going to put your plants so they can get their roots down past it.
Lift the first few layers of concrete up one by one, and fill with soil or potting compost underneath in the gaps between the concrete. Try to fill most of the gaps down almost to the soil base, so plants can get roots down, but leave some air gaps in the lower layers to allow for free drainage.
Once the lower layers of concrete slabs have been piled up with soil in between, some care must be taken with placing the final layer. Place the slabs so as to leave decent size gaps between them in the spaces where you want to put plants, and fill these up with soil. Make sure to leave a crack between the slabs underneath where the plants are going, so they can sprout roots down to reach more soil. Water the whole mound and allow the soil to settle, filling in with more soil in any areas where gaps appear.
Take the bulbs or other plants and put them into these prepared spaces, then cover the roots with soil, and mound some more soil above the root ball to protect it, leaving the shoot sticking out. You may need to water again at this stage to make sure you have added enough soil once it all settles down.
Carefully place the last layer of slabs above the plantings so that the shoots you have left stick out of the cracks between them, and then cover the pile with more soil, and water again. The dense roots from the main plantings tend to prevent weeds being able to sprout through from underneath, but to prevent weeds growing in the top covering layer of soil it is advised to plant a shallow rooted ground cover to fill up the space.