Answer: Baesd on your description of the problem I am not sure exactly what is happening to your plants. You may simply find that your lilies are suffering a nutrient deficiency or are suffering from hot dry summer weather. However, lilies are occasionally subject to viral and disease problems, so it is a good idea to clean up and remove any plant debris as it happens to try to avoid spreading the problem. In a hot dry season as we are experiencing this year it seems all the possible problems are worse than usual.
The late/non emergence in spring would lead me to suspect that either an animal such as a vole snacked on them over the winter, or that the drainage is less than ideal (they absolutely require perfect drainage, especially if the native soil is a heavy one) or that the bulbs were not sustained by last year's foliage growth after flowering. It is also possible that they were not planted deeply enough in the beginning and thus froze out or heaved out.
These plants do best in full to half day sun with rich soil and perfect drainage, planting on a slight slope is ideal. The old saying is to put their heads in the sun and their feet in the shade. An organic mulch to help hold down weeds and retain soil moisture is useful and an annual topdressing with compost and/or a balanced fertilizer is also helpfu for maintaining vigor. Air circulation can help with disease problems; crowding alone can also result in less vigorous growth.
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