In our small back garden we have a little (12ft 7ft) veg plot which we
top up by dumping our home-produced compost on every winter.

To the rear of the veg plot are some lopped 8ft tall conifers (5-10"
trunks). Although the veg plot is raised 6", by the end of October it
contains a lot of conifer roots.

Moving the veg plot/conifers is not an option. Would digging down 18"
and laying some of that weed suppressing black material stop the roots?
Would they still take a lot of moisture from the soil?

Any ideas greatly received?

Thanks.