Answer: Roses are large, long lived plants so it is worth digging a very generous sized hole. It should be several times larger than the root system of the plant you are putting into it, as a minimum. A hole two to three feet across and would not be too big. If the plant is grafted, it should be planted with the graft several inches below the soil surface. If you just planted them, you can certainly reset them deeper if needed.
Roses need a well drained location so sandiness is not such a bad thing. You should however add a generous amount of organic matter to the soil. This will help the soil hold moisture and nutrients rather than drain quite so fast.
There are many kinds of organic matter and they can be mixed. Milled spagnum peat moss (not peat humus), good quality compost, well rotted autumn leaves, well aged stable manure and bedding can all be used.
After planting, use an organic mulch year round. It should be several inches thick in a flat layer over the root area. This will help add additional organic matter to the soil over it time as it breaks down. Good luck with your roses.
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