Yikes. This is the worst garden soil I have ever seen! There is maybe 10-15 cm of topsoil in this garden, and underneath is pure Calgary clay. Whoever landscaped this yard ripped the homeowner off. Fortunately for me, it's not my soil. But what to tell my client?
There are two ways to improve garden soil. A good 30 cm of topsoil is required for plants to really thrive. 45 cm would be even better. The best way is to get this is to get a Bobcat in to remove the existing soil down to at least 30 cm and have it replaced with good black loam. This is great if you are starting from scratch (and if you are, then I highly recommend doing this! You will not regret it), but if you already have an established yard or just want to add a few plants here and there, it's not very practical.
The other option is to improve the soil as you dig each hole and plant each plant. Faster in the short-term but possibly more work in the long-term. So, in this garden, I dug each hole 3 times bigger than the size of the pot that the plant was in (what a workout!), and mixed lots of compost with the existing soil in each hole. This will make space for each plant to grow its roots into without struggling in the hard clay. Top-dressing the whole garden with 2-4" of compost each spring will also gradually help improve the garden soil.