Answer: It sounds like transplant stress, coupled with too much water. Whenever you plant in the mid-summer heat, you'll have to expect the plants to look stressed for a few weeks. Once their root systems become established they will perk up and replace those lost leaves. Most landscape trees and shrubs need about an inch of water per week, applied slowly so the water penetrates deeply. This will encourage deep rooting which will help your plants endure hot summer weather. When you water frequently you keep the soil too moist, which drives out the oxygen. Plant roots need oxygen as well as moisture; without oxygen they will suffocate. My advise is to water thoroughly and then wait 3-4 days. Check the soil moisture by digging down near the roots of your plant. If soil is still moist 2" below the surface, you won't need to water for a few more days; if it is dry 2" below the surface, it's time to water. By adjusting your watering practices, your oleanders should regain their health in a few short weeks.
Best wishes with your oleanders.
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