No, that's not a steep slope; you don't need a retaining wall.

Thanks! So i just plant the tree straight in the ground?

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Just keep the drain clear of the fallen leaves. You don't want a drainage problem near your house.

Just plant your tree in the ground, but hopefully it won’t grow very large. That’s not a very big spot and it’s closed in by a fence and concrete walkway and what looks like a driveway. You want to be sure you can drive/park a car there.

Do you own the land on the other side of the white fence? Obviously a mature tree will hang over it.

You are better off without a retaining wall. Shrubs are a good way to keep dirt from eroding on a slope. In that space, you want a smallish tree. Make sure your tree is suiting for your site (sun, climate, soil). Be sure to keep it well-watered in the first year. (If it's suited to your climate, after that, it should be okay with just rain, unless you have an unusually dry period).

In that small space, I would go with a shrub or small tree, and follow Sigrid's advice about choice and care.

If you plant a tree you run the risk that when it gets larger the roots will undermine the driveway and sidewalk paving. If you do move forward with a tree, talk to a nursery (not a big box store) about what kind of root system the tree will develop over time and whether this will be a concern. Bushes are a better choice. You could have a nice planting area there. Do you know why the grass hasn't done well there?

Modern Prairie Front Landscape - New Berlin, WI · More Info