In everyday language, any plant with an underground storage organ is referred to as a bulb, even at garden centers. There are many categories, technically speaking, with different names for different structures.
True bulbs, such as tulips, are made of modified leaves that are attached to a flat basal plate. They surround the following season's flower bud. Some bulbs (such as daffodils and tulips) are surrounded by a papery tunic, and some (such as lilies) are covered by fleshy scales.
Corms look much like bulbs but when cut open, they have a solid starchy interior stem. Crocuses grow from corms.
Tubers are modified stems with starchy interiors but no basal plate or tunic. Both roots and shoots sprout from the same growth buds, called eyes. The potato is a typical tuber. Tuberous roots are similar but are actually swollen roots, not stems. Dahlias produce tuberous roots. Rhizomes are thickened underground stems. They grow in a horizontal manner, sprouting new sections as they spread. The bearded iris is a typical rhizome.
These categories of a bulb define the structure of your flowers and will determine how you eventually map your landscaping plan.
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