Please let me know the best way to have this plant grow quickly and dense/bushy.

==>> leave it alone for 1 to 3 years ... and then start shaping it into what you want ..

every leaf is a food making machine.. and the plant needs to grow the root mass.. to grow the plant you want ...

with the grass looking as it does.. i would NEVER feed them ... but the stuff you have.. is so benign.. a LITTLE of it wont hurt them ...

your only problem here.. that i see ... is your wish for instant gratification ... you need to wrap your head around shrub or plant time.. and give them time.. to grow the roots you will never see ...

just water properly ..see link

you planted a bit close to the fence ... i wouldnt mind taking a few branches that start going thru the fence ... earlier than i mentioned above ...

i suppose the time frame may be faster in FL.. since you have a long.. if not year round growing season .. but you still have to be patient ....

proper .. deep.. in frequent watering is the key ... insert finger and find out how your watering protocol works in your soil ...

ken

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

I'm told that these plants can grow a few feet a year in South Florida. However, I want to make sure they are thick and dense as they grow and expand horizontally so that they start blending into one another.

I was also told that you can create more branches and a fuller hedge by "pinching" the stems off the top of many of the umbrellas, but I couldn't fully understand how this is done, and exactly where I need to cut off the branches, etc....that's why I was hoping for a better description or maybe some type of pictorial guide.

However, if just leaving them alone truly is the best method for them to grow dense, taller, and bushy, then I will do that. I will add a little fertilizer in a few weeks just to help with the growth.

In terms of watering, I have no control over that. It rains a lot in the summer, but I live in an HOA where the irrigation is controlled by them. The sprinklers come on every night, and I have no control over them since it is run by the HOA.

Anyone else have any input or suggestions?

I only know these as houseplants but, firstly, there isn't a way of making them both taller and bushier at the same time. The process of making them bushier makes them (temporarily) shorter. To get them bushier cut off stems just above the point where a leaf joins. Where you cut two new stems will grow. You can do this wherever you like on the plant depending on where you want the new growth to sprout. Once you have the new growth you can just keep repeating the procedure until you have the density you want. Then you you can let them grow taller. But you'll need to keep pruning to retain the bushiness at whatever height you decide on. It's not complicated and these things do grow easily. Any pieces you cut off can be rooted as cuttings if you want more plants. If the hedge is thin at the base you could even stick the prunings into the ground there and get new plants.

I can't help you with the fertiliser question as I have never needed to use it on shrubs in the ground. Judging by the lushness of the lawn the hedge isn't going to need any further help.

do what ken said, you must train them slowly