Answer: Keep up your normal watering routine for both your irises and your lilies all summer long. The foliage on the lilies is manufacturing carbohydrates and transporting them down to the bulbs for storage. The more energy the bulb stores, the larger the plant and the more flowers it will produce next spring. Once the bulb has collected as much as it can hold the foliage will begin to yellow and then turn brown and die. Keep watering until it begins to turn yellow.
Irises will remain green all summer and on into the winter. Water as usual until the fall rains begin. At that time you can stop watering.
Best wishes with your garden.
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