Daffodils contain chemical compounds (crystals) that make the bulbs unappetizing to squirrels and other rodents. These compounds do not, however, make the bulbs allelopathic or toxic to other species growing around them. In fact, interplanting tulips and narcissus may confuse rodents into avoiding the tulip bulbs.
Interplanting tulips and daffodils make sense when space is at a premium or in cottage-style gardens, where many species and varieties co-exist in one area. Tulips and daffodils can also be planted together in one large container for a dramatic spring show.
Most bulb purveyors provide information about bloom times for spring-flowering bulbs. When planting tulips and daffodils together, pick varieties that bloom at the same time, for lots of color; or in succession, to prolong the spring show.