Coffee Grounds

Kathleen,
The purpose of coffee grounds is to help acidify the soil or help lower the PH of the soil and it will help retain water. It all depends on what type of soil your using and what you are trying to grow. My brother recently added coffee grounds to his soil for his gardenia. Not all plants are acid loving plants like hibiscus. To do it right you would need to take a soil sample around the roots, make a slurry and check the Ph to determine if you should lower the Ph of the soil. I think there is a topic on how to make the slurry to check the ph. Personally, I wouldn't add coffee ground because there are many different ways to help acidify the soil besides coffee grounds. Coffee can get rancid and rot to. The PH of the soil is very important for nutrient absorption and something to consider if making your own soil. The problem with making your own soil is if you don't know what your doing, you could kill your hibiscus and by the time you buy everything that is needed, your spending the same amount of $ as if you bought hvh potting media. I know shipping can be pricey but I can sleep at night knowing my soil is ideal for hibiscus and not worry about PH.
Just because you watered your plants doesn't mean they are wilting because of lack of water. They could be wilting because you over watered and the roots can't get air. They could be wilting because the direct sun is to strong and they are not completely acclimated yet. They could be wilting because it's to windy. They could be wilting because they have wilt disease. They could be wilting because roots are showing and baking in the sun. They could be wilting because they are in to small of a pot.
I can't tell you if you should use trays or not. This is where you need to determine if you should use trays. Not all trays are the same. It really depends on the situation. Some of my plants have trays some don't. I might use a tray on Monday but not on Fri. It all depends. If you don't have any beneficial's in your soil, of course it's going to dry out half day in FL sun. If the soil dries out to much, the next time you water, it may run right out the pot and not have a chance to soak the soil. This is when trays come in handy. Small pots like 4" pots do not hold much water and when you do water it can run right out the bottom and not give the roots a good soaking. If you use trays don't let the plant sit in a puddle of water for over 12-24hrs because that is poor drainage. If you see the hibiscus has not drawn up the water in the tray, it could mean the plant is not thirsty enough and can't absorb anymore and you may need to spill it out. The problem with trays is people over water and let there plants sit in a tray of water for to long. Or it rains so hard the root ball is soaked sitting in a puddle of water for days. Try to be very observant if you use trays and watch to see if the plants are absorbing the water in the trays on there own. For me trays are helpful but I know how to use them and when. If you use trays, your also at risk of burning your plants because they draw up fertilized water. It depends on your soil but large pots don't usually need trays since large pots can hold enough water to get the plant through a hot day. Some people use trays because they don't want there deck or floor stained.
Hope this helps,
Kyle