move away some of the gravel beneath the sick plant and make sure the soil is moist to several feet deep. If it's not moist, get out the hose and water the whole area deeply (like about 55 gallons or so) every 3 days.

Drip emitters are not maintenance free. They clog up after a while with our salty water.

good luck.

could be too much water as well. yellowing leaves is a sign of it.

OK, lets do a little math. Let's say you have a one gallon emitter on each plant. You're watering 90 minutes, 3 times a day. That's 4.5 gallons each time you apply water (assuming you don't have clogged emitters).

A three foot shrub needs about 8 gallons of water each time its' applied. Visualize that one gallon of water will moisten one cubic foot of soil. 8 gallons would moisten an area 2' x 2' by 2 feet deep. That's just about right for your plants now but as they grow they'll need more water. Watering deeply pushes any accululated salts below the root zone which can cause yellowing leaves.

A better strategy for watering is to apply the water once a day rather than 3 times a day. Allowing the soil to dry a bit helps oxygen return to the soil - roots need oxygen in order to absorb nutrients from the soil. Every three days this time of year is probably OK for your hibiscus. Desert adapted plants can get by with once a week watering.

Instead of rock mulch, have you considered using a 3 inch layer of organic mulch? It does several things. 1) slows evaporation; 2) keeps soil cooler; 3) releases nutrients back into the soil; 4) reduces reflected heat and light on the plant.

Hibiscus are acid-loving plants and they struggle here in our salty soils. Once a week or so use a teaspoon of white vinegar in a gallon of water and apply it to your hibiscus. This will help neutralize the soil a bit and make your plants happy.

Good luck.

Hibiscus is a tropical plant, not a desert adapted plant. They can't take the heat of a south facing wall and they need afternoon shade. My are planted on the north side of my house and receive afternoon shade from the porch. I amended my soil by adding a lot of peat moss and compost. I drip 2 gallons per plant every 3 days. You don't want the soil to stay soggy or the roots could rot. The only hibiscus I had success with is San Diego Red.