How do you know when spring has begun? Is it the appearance of the first tiny leaves on the trees, or the first crocus plants peeping through the snow? The First Leaf and First Bloom Indices are synthetic measures of these early season events in plants, based on recent temperature conditions. These models allow us to track the progression of spring onset across the country.

The First Leaf Index is based on the leaf out of lilacs and honeysuckles, which are among the first plants to show their leaves in the spring. This Index is associated with the first leafing of early-spring shrubs and other plants.  The First Bloom Index is based on the flowering of lilacs and honeysuckles. This Index is associated with blooming of early-spring shrubs and leaf out of deciduous trees. 

The Sacramento Valley as well as coastal areas of Northern California, Oregon, and Washington are seeing the earliest spring leaf out on record.

https://www.usanpn.org/data/maps/spring

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