After the foliage has turned from green to yellow or brown, cut down old stems to ground level and discard in the trash or bury the stems deeply in the garden. Avoid adding old stems and lily leaves to compost piles as this could potentially aid in the spread of Botrytis
fungus the following year. Winter compost heaps usually will not become hot enough to destroy fungus spores; burying or trash disposal is recommended. This is the same technique used by rose growers to keep "blackspot" and other fungus issues in check.
If desired, give a top dressing of one to two inches of well-rotted manure or finished compost over the top of your bulbs for a light feeding in spring and to reduce weeds next summer. Remember those "feeder roots" just between bulb and soil surface? Basal plate roots at the bottom of lily bulbs are to anchor the bulb, not primarily for nutrient uptake. Roots near the surface are the most important, why we do not recommend mixing fertilizer into planting holes, it can leach out over winter and be wasted. Avoid pure peat moss for mulch; when the surface dries, it repels water, but a mixture of peat and manure or compost is fine. In colder climates (especially if adjacent to vacant lots, fields, or woods)
please wait until winter has firmly arrived or there has been several bouts of freezing weather before you mulch lilies. If you mulch too early before the ground is frozen, pesky rodents may take up residence in the soft soil just under the mulch, consider your bulbs a gourmet treat left for them and feast all winter.