You have a two position boot floor - if you are at all concerned, rather than pack the space under the boot floor, simply move it to the lower position so that there is no space beneath.
That said, I have the boot protector in place which wouldn't fit with the boot floor lower but have no concerns at all about the cars ability to carry 'normal' loads. While you might have a problem if you placed a large concrete cube in the centre of the boot, normal bagged stuff from the garden centre or DIY store can easily be distributed across the available floor space. And, in the grand scheme of things, horticultural bark isn't that heavy really.
That said, it's an estate car rather than a pick-up and you shouldn't attempt to drive with an excessive load onboard.