I have yet to see any kind of barrier that works really well to keep grass out of a planting bed because the grass will often simply grow over that barrier if it is not, periodically, raised to prevent that. Work on cleaning the edging around that bed and cover the grass, crabgrass included, with newspaper and cover the newspaper with a good mulch (to hide the paper and help hold it in place) and don't worry about any poison to control that grass.

You need to remove the metal edging. Kimmsr is correct, there is no effective barrier. Any form or artificial edging will just give you a false sense of security.

you then need to cut in a deep v shaped trench along the line in your photo. Hand weed out the grass in the tree bed.

Religiously maintain your cut edge to the lawn. Trim it once a week without fail, preferably with sharp long handled lawn edging shears.

Here is a link that might be useful: Bahco Lawn Edge Shears

A trench canbe an effective barrier to grass growing into a flower bed, if your soil allows that barrier to exist. Sand does not and will simply flow back into the trench about as fast as you cut it. In clay soils it can work and can be very effective, but like mulches some people do not like the look.