Answer: You really shouldn't cut back the perennials early, even though it makes it difficult to plant the bulbs, because it may stimulate the perennials into extra late season growth. If you are planting close enough to the perennials that overhanging leaves are a problem, you may be trying to plant too close, and planting bulbs will disturb their roots. Remember, too, that most bulbs multiply as they grow, and your bed will end up very crowded. Bulbs that may be treated as annuals, such as tulips, will cause soil disturbance when you remove them in the spring, too.
There is no perfect answer to your second question. Some gardeners simply do the best they can and plant the bulbs where they fit in, using an informal scheme or drifts rather than a strict bedding pattern. Sometimes you can plant the bulbs toward the back of the bed and leave that area free of other plants. Or, you can plant the bulbs in an area where you will plant annuals the following summer, or allow annuals to come up from seed in that area to conceal the bulb foliage -- which can look pretty ratty -- while it matures and fades. In order to return and bloom well each spring, the bulbsl need to be allowed to grow and ripen their foliage. In other words, unless you designate a separate area for bulbs, just do the best you can!
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