Does anyone know whether this software has the ability to accomodate rectangular gardens with sides of different lengths? Trapezoidal, for instance? Thanks.

Plangarden gives you the building blocks to create many odd shapes.

The way to build a trapezoid is to first place a square and then put to triangle next to either side of it.
Use the measuring tool to get the dimensions to match the ones in your garden.

Let me know if this helps. If it doesn't, I am happy to go into your garden and create on for you.

Thanks,
Roy