I was picturing something far more dramatic than that from the original post! There's no need to worry. If you're establishing a new shrub or plant, you can scoop a horseshoe of soil around it on the downward side, to 'dam' the water when you are drenching it. Once established, plants should be OK. You might find that plants that like a moister soil do slightly better at the bottom of the slope, and plants that prefer good drainage prefer the top. But it's not so steep that rain won't get a chance to soak in, by the look of it.
"What is hateful to you, do not do to your neighbour".
You have a flat garden, and nothing needs to be done to it. I'm on London clay, and there is an 8 foot drop between my top terrace and patio. We don't get flooding, or torrents of water, even with such a steep garden.
It would be great to be able to see how water moved through slopes. I was told by a gardener friend that nutrients tend to flow down.
My garden feels vertical some days. You can see over the house not that far up the garden, which gives you an idea of height.
Once there is a good root mat it's amazing how well the top soil is retained. Even with torrential rain. Mulching helps. I had a pile of fine soil piled last year, revisited it and a sycamore had happily moved into the entire pile and it was pretty solid.
The biggest challenge for us is getting water up the garden. So I have thought about digging rain catching gullies at points in the garden for hard run-off catchers.
It's ridiculous that in South East England even when it feels like we've had continuous rain we can still be in a drought. The garden goes from one extreme to another. It is currently a dust bowl through lack of rain.
Posts
Your garden is lovely!!