Answer: There are several possible explanations as to why, most of them related to microclimate. A sheltered location out of wind and with reflected heat will bring things along faster and can even add a half a zone of hardiness. In some ways this is a benefit because spring comes earlier and winter a bit later. To take advantage of it, you need to expand your plant paletter to add very early and later blooming plants.
The reason they last a shorter period may be a combination of the heat (the season extending properties also concentrate the summer heat) and possibly your watering routine. When plants are heat stressed they need more water, so it is important to make sure they are not stressed for water. Improving the soil by adding organic matter, watering deeply at longer intervals rather than a daily sprinkling, maintaining a summer mulch to keep the soil cool and moist, and selecting plants known for heat tolerance will all help.
I hope this gives you some ideas of things to experiment with.
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