Answer: The very best time to dig up your lily bulbs would be in the early autumn, after the foliage has turned yellow. Be sure to protect fall dug lilies from drying out or freezing if you can't get them replanted immediately; roots will still be fleshy. Lily bulbs do not go fully dormant and must be protected from drying out at all times.
If fall division is not practical, the earlier in the spring you dig them, the better, although you risk damaging the bulb since you may not be able to tell where it is. If you wait until summer when plants are approaching mature size, you risk stressing the plant. When digging a mature lily, ideally you should try to dig and replant it immediately, keeping soil in tact around the bulb and roots. If you can't replant immediately, keep lily plants in a cool shady place, and don't let them dry out before replanting.
Plant oriental and trumpet lilies 4-6" deep and 12" apart, and Asiatics 3-4" deep and 8" apart. Divide garden lilies every 2-3 years, based on their performance. Discard any bulbs that are damaged or rotting (place in the trash, not the compost). Separate the bulblets (small bulbs) from the stem, and plant them in a nursery garden for a year or two until they are mature size, to multiply your collection. Bulblets should be planted at a depth equal to three times their height. As lily bulbs grow in size they pull themselves deeper into the earth. When it is time to move your offspring, you will have to be careful to dig deeper than you planted them to avoid damage.
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