Protecting Pennyroyal over Winter

How should I protect a Pennyroyal ground cover during the winter as it is considered a tender perennial. We live in a suburb of Ottawa and I believe that because of the cold weather experienced in the last few years, Ottawa may be classified as zone 3.

Pennyroyal is hardy in zones 6 to 9. We have found that it does not winter reliably in our zone 5, so we are doubtful that you will be able to get it to winter in your zone 3. Often you can "push" plants one zone, but three zones is too much. However, there is no harm in trying. The factors that affect hardiness are complex and interdependent, and it is possible that you may be able to conspire to create a microenvironment that pennyroyal can survive in.
Things to try. Make sure the water drainage of the soil is excellent. You improve drainage by adding sand to increase the porosity of the soil and by planting on a slope so water can drain away from the plants. Pennyroyal, like many herbs, suffers from winter freezing and thawing cycles. The less water around the roots the less damage that will occur.
Anything that keeps the ground frozen during the late winter period when the ground thaws and freezes frequently. Experienced gardeners in northern areas pray for a good snow cover to help keep the ground frozen. Laying a mulch of leaves or branches on the garden in the fall achieves much the same effect, but remember to remove the mulch in early spring.