Alma, we're equatorial here, so the weather goes a bit different to where you are. Our lowest day maximums and our highest night minimums are in summer. Winter the day maximums are higher, night minimums lower. Spring has hottest days and hottest nights. Autumn isn't usually quite as hot as spring. This is all on average, of course.
It all revolves around our rains. Winter is dry season so hot and sunny but lower humidity. As summer gets closer (spring) it gets hotter with much higher humidity. But summer is the rainy season so the rain cools it down. Although sunny days during summer can get very hot and humid. Autumn the rains ease off so it gets hotter again
So we're in heat zone (NOT cold zone) 14, meaning 350 days per year of equal to or over 30C (86F). But average temperatures are increasing here as well. It appears to be a worldwide trend.
The fact that the Vinca grew okay before I heavily mulched does make me suspect that the very poor bare soil in the newer area was more of a culprit in its demise. Also, my soil won't hold water. The irrigation is set to twice daily for much of the year, after sunset and just before sunrise. But that's because I work away from home for a couple of weeks at a time.
