The United States Department of Agriculture (USDA) plant hardiness zone 4 has average low winter temperatures that can reach between minus 30 and minus 20 degrees Fahrenheit. Because of the extremely low temperatures, gardeners in these climates often resort to flowering annuals instead of perennials. Fortunately, there are some perennials, including many lilies, that are hardy in zone 4. Many lilies in fact, grow and bloom well each year from the middle of June until the middle of September, even those that are not winter-hardy in the area.
To know when to plant your lily bulbs, know what kind of lily bulbs you have. There are many types and each has its own planting needs and times at which they should be planted based on whether they are hardy or tender in your area. In addition, some plants called lilies are not true lilies, such as peace lilies, and this greatly affects when they are planted since they may not be hardy in your area as you might expect.
In addition, be sure you still live in zone 4. The USDA makes adjustments to the hardiness zones on occasion as the climate changes. Your area will remain in zone 4 as long as winter temperatures remain about the same.
Plant lilies that are labeled hardy in zone 4 (e.g., Asiatic, Oriental) in the fall around the first frost, usually between September 15 and October 15. Then just before the ground freezes (usually in November), cover the area with 4 to 6 inches of organic mulch. Example mulches include compost, wood chips, pine needles and leaves. Be sure there are no weeds in the mix. Mulch will help insulate the ground so temperatures do not fluctuate too much and it will help delay the ground from freezing so the bulbs have more time to grow roots.
Plant lilies that are not hardy in zone 4 in the spring (e.g., Easter, Lilium longiflorum) after the ground has thawed and it has warmed up a bit. This may not be until mid May or early June in zone 4. Look at the lily bulbs label to see if there is a soil temperature at which lilies can safely be planted. These lilies will then need to be dug up and stored indoors during the winter months each year to bloom again.