Roses are growing and flourishing after their winter haircut. New leaves often mean powdery mildew and black spot. The Cornell formula is a less toxic method of control, but you need to start applying it before you see signs of disease.
Add 1 tablespoon each of baking soda and oil (either a light horticultural oil or regular vegetable oil) to 1 gallon of water. If you use vegetable oil, add a bit of insecticidal soap (the amount recommended for mixing with 1 gallon of water). Shake well before and during application with a sprayer. Spray both sides of the leaves thoroughly every five to seven days.
Antique and English roses are more resistant to these diseases.
Hibiscus plants should be flourishing with new growth. Plants that were injured by cold temperatures should be recovering nicely. There is still time to prune if you need to shape your plants.
Magnolia trees are called broad-leaf evergreens but they do shed their old leaves during spring. These old leaves turn pale green or yellow and drop. Trees do not become entirely bare. Occasionally a magnesium or iron deficiency will cause yellow leaves but they have a distinctive pattern that can be identified easily.
Source: Theresa Badurek, urban horticulture extension agent, UF/IFAS Pinellas County Extension Service. Learn more at askextension.org or pinellascountyextension.org.