Flaked rock has sharp edges and makes a more solid barrier than gravel. Surround each bulb with a few inches of this material. The lazy voles will not dig through the flaked rock and the bulb shoots will grow right up through it. If you are plagued with voles, you will have to resign yourself to more frequent maintenance and dig, divide, and re-plant your bulbs every few years. Your only other alternative is to hope for the best, that the voles will miss a few bulbs, allowing them to grow and flower.
Lilies are wonderful and dramatic additions to the garden.
While a bit expensive initially, if planted in good, well-drained soil in mostly sunny locations, they will increase every year and more that repay your investment with larger and more beautiful shows. It is well worth the effort to seek out reputable dealers and buy the best and biggest bulbs you can find. As with other flowering bulbs, the size of the flower is determined by the quality and size of the bulb. Puny, dried out specimens may not even grow let alone flower the first year in the ground. Good bulbs, properly planted, will reward you with years of trouble-free beauty in your garden.