Unlike shrubs, perennials generally do not have woody structures.  Generally here means that there are always exceptions.  Some perennials may develop woody bases with age, and this helps protect them from predators or winter cold.  Instead of being defined by their woodiness, perennials are classified as herbaceous plants with tender stems vulnerable to frosts. Encyclopedia Britannica defines an herbaceous perennial as a plant species that dies back to the ground over winter.  While that is indeed the case for many perennials, when/where did this definition come from?  Most likely from a place of harsher winters than California's Mediterranean climate.

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Many perennials in California, particularly from our Bay Area, Central Coast, and Southern California regions, experience mild winters with few frosts - a phenomenon becoming more apparent with climate change.  Therefore, some herbaceous perennials that might die back in other regions will instead retain their leaves and structures in California.  We can find a very good example in daylilies, or scientifically, Hemerocallis.  Daylilies are indeed herbaceous perennials, but they can be subcategorized as both evergreen and deciduous varieties.  Evergreen varieties will retain their leaves all year, whereas others will go semi- or fully deciduous over winter.  How disappointing would it be for a designer who relied upon the winter greenery found instead they specified a variety that dies back to the ground?  Watch out for Salvia, commonly known as sages, because this genus covers shrubs, perennials, and even biennials and annuals (only lasting two years or just a season, respectively).  Why is this important?

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As landscape designers, we rely upon our plant selections to help tell the story of our designs.  We use trees as roofs and shrubs as walls, for example.  Perennials create a nuanced air of elegance, reinforce design styles, may provide a punch of color and texture, or support benefits in the home and kitchen, but they can completely disappear over the winter, too.  How does the designer's composition hold up during this downtime?  Will the perennials only add value for a very short time, such as during a two-week bloom period in spring?  These and other questions will need to be considered when choosing perennials.

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Perennials for the summer or autumn garden may produce flowers, fruit, or even unique seed structures late into the seasons.  Their details need to be explored by designers, as often exploited in wonderous ways by Swedish designer Piet Oudolf, or historically by English garden designer Gertrude Jekyll.  When taking a plant identification class, pay close attention to the seasonal timing of leaf and flower color but also fruit and texture.  Even in their winter dormancy, their structures may provide interesting forms to enjoy through the seasons.