Beginner Gardening: Mandevilla drops 80% of its leaves last two years in late Aug/Sept??, 1 by MitchinCali

MitchinCali wrote:
Hello Community! Advice? 4 year old Mande has grown like crazy. I am in 10A. It is in a 18X18X18 pot with trellis. Sits outside kitchen window year round facing the 10am to 4pm full sun. March to July grows like crazy, deep green leaves, tons of flowers, vining like crazy. I water it every 8-10 days or so in cooler months, twice a week in spring, 3X a week in hot summer. Give it some properly diluted Miracle Grow every other month. The heat of Aug comes (100-105 daily) and it stops growing. The leaves slowly turn yellow and drop over next 6 weeks until now. It loses approx 80% of its leaves. It has some new growth, but very tiny. This thing goes from a green leafy beast to a scrawny sick looking vine! It did this last year but it came back strong in spring...now here we are again. I thought these loved full sun...is this too much sun? What am I doing wrong?