I have about 7 brugmansias, some in large pots, some in the ground. All, for years, have lost lower leaves and most are now just tall & ugly with no bushy-ness to them. Yes I fertilize but probably not enough. I'd love comments.
Keeping them too wet will make them drop lower leaves. Mine lost a lot of leaves this past year due to white fly. If they are leggy, I would cut them back by about two thirds and let them start over. Then root the cuttings and you will have more. I dont dont fertilize, per se, but I top dress twice a year with compost.
They need some direct sunlight to stop reaching higher and regular weekly to biweekly fertlizer in order to grow good. They will lose leaves everytime they become stressed by either drying out too much or being overly wet. They don't do well with neglect. Lots of light, steady moisture not overly wet or too dry and heavy doses of fertilizer, keep the bugs off and you will see blooms and lush foliage.
Look for a fertilizer with a 3 : 1 : 2 ratio. Something like hibiscus fertilizer is close to ideal, but specialty fertilizer are more expensive. MiracleGrow works just as well and comes close. You want to stay away from using fertilizers that are heavy on phosphates. Heavy phosphate use tends to yellow the older leaves which then drop off leading to more leaf drop. Brugs are heavy feeders. To keep them growing and producing those large leaves and heavy flushes, they need to be fertilized at least once or twice a week (If growing in pots. Slightly less if growing in the ground.) when they are growing actively.
I noticed you didn't answer the questions put to you about the amount of direct sun that your Brugs are getting. With the exception of some highly variegated Brugs, they need several hours of direct sunlight, preferably in the morning. Without seeing photos of your Brugs and not knowing any cultivation history, it is difficult to say what is going on with your Brugs. Leaves do age and fall off with some regularity even in the best cared ones. New branches and leaves have to be encouraged to grow farther down on the branches by judicious pruning, fertilizing, watering and making sure they get some sunlight in the morning and plenty of bright light for the rest of the day.
Are the Brugs having to complete with other plants? This could be a problem especially if they don't get much fertiizer. You can renovate the ones you currently have in the ground by pruning as suggested by steadycam. If the soil is compacted and depleted of organic nutrients. You might be better off pulling them out and conditioning the soil before replanting the pruned plants. Potted Brugs need to be re-potted every so often. How often depends on the size of the pot they are in. If they are in 25 gallon pots, they could remain in the mix for a year or two. Plants in smaller pots need repotting at least once a year. Potting soils do break down and need to be replaced. Some old anchor roots need to be removed to provide room for feeder roots to form. So root and top prune those potted plants. Plants growing in the ground take up nutrients and lock them up in the top growth eventually depleting the soil. If we expect the plant to continue growing, we need to provide those nutrients either by fertilizing, adding compost or both.
In areas that seldom get hit by frost or heavy freezes, Brugs can become quite tall. I remember cruising through some Palo Alto neighborhoods and seeing Brugs that were taller than houses. They are after all small trees/large shrubs. It's just that most of us try to keep them smaller.
No. If you pinch the tips of vegetative branches, you delay the formation of the first "Y" for that branch. Pinching flowering branches isn't necessary. The way flowering branches grow each time branches produce a new "Y", they also produce flower buds. The best thing to do is to make sure you are fertilizing regularly.