glyphosate should work and you can spray brown wood ( tree bark ) as long as you don't get it on the leaves but be sure to read the instructions carefully and make sure that there are not some little gems of plants lurking around

Or strim off the weeds and cover the area with black plastic/landscape fabric and wait for the weeds/lawn to die off. When you remove this layer then you can dig it over. Although this is only suitable for a smaller area.

The black plastic is likely to damage the plum and apple if it's applied within the drip line of the trees. Also, apply just enough glyphosphate to wet the leaves of the weeds: it has a surprising amount of soil action if the soil is drenched with it.