Answer: There are possibly several contributing factors to why your plants aren't blooming until alter. These plants need plenty of sun and very rich soil and ample water in order to grow their best. They also need plenty of root run area as they
grow larger. Potting soil can become depleted over time and they can truly outgrow their pots. You might see if you need to repot them into larger pots or consider (next spring) cutting them back, root prune them and repot into the same size pots. Ideally, this would be done annually in early spring before they launch into their summer growth spurt during their outdoor vacation. It is also possible that the plants are still rebuilding their strength before they bloom -- a winter indoors is always a bit stressful for sunloving tropicals -- and as a result they are somewhat seasonal in their blooming. They also have a tendency to bloom better when temperatures are warmer. If you have a cool summer that might be an influence. Finally, many plants flower off of "new" wood; while some bloom on "old" wood. I couldn't find a specific reference for how mandevilla blooms, but if it blooms on new growth and you are cutting back the new growth, that would reduce bloom. You might try letting the green shoots grow. Pinch them back occasionally if you want to promote bushiness, but then observe where your plant blooms next summer. I hope this info helps!
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