I've planted a Bougainvillea. It was doing great, then suddenly it turned black and died. So I bought two more plants last spring, and they too flourished. But now, they have turned black and died. What is wrong?
Cold-weather or frost is the primary suspect. Bougainvillea needs to be protected from the cold when the outside temperatures reach the mid-thirties. Bonide Wilt Stop, a plant blanket, mini, non LED lights are methods of protecting cold-sensitive plants.
The Bay Area is a mixture of microclimates. Winter temperatures are warmer the closer to the water you live in and much colder in the inland protected valleys. Bougainvilleas survive the winters in many areas; however, not every winter. Every twenty years, we have a freezing period that kills the plants.
Besides Bougainvilleas, Citrus, Hibiscus, Mandeville's, other tropical plants suffer and die from freezing temperatures. There is a chance the plants would come back next spring, so I wouldn't be quick to pull them out.
I wouldn't prune them either, as the damaged foliage will protect the rest of the plant during another cold spell. Instead, I'd wait until April for the flush of new growth. If none develops, I would replace the Bougainvilleas with something else or continue to replant, expecting them to last one season. The nursery professional at your favorite garden center is an excellent resource for alternative vines.
The leaves on several Camellia have turned yellow. This is because they're growing behind a retaining wall in a sandy soil mixture, not the traditional adobe clay. I feed them periodically, and they get morning sun. What do I need to do to get the leaves green again?
If the leaves are uniformly yellow with none of the green veins visible, I suspect the plants are just hungry.
Unlike adobe soil, nutrient retention in sandy soil is very poor. Sandy soils are made up of large particles. These particles create large pore spaces, so nutrients quickly leach out of the root zone when water is applied. In comparison, Adobe clay is comprised of very tiny particles.
The primary nutrients for plant growth are Nitrogen, Phosphorus, and Potassium. Nitrogen gives plants their green color and is very mobile. Phosphorus and Potassium are for fruit, flower, and general plant hardiness. They're non-mobile elements.
Nitrogen moves or leaches out of the root zone quickly with water. A Camellia is a shallow-rooted plant with most of its roots near the soil surface. The loss of leaf color is one of the primary characteristics of Nitrogen deficiency. In sandy soils, plants require frequent fertilizing.
You'll not see any changes soon because the growing season has concluded if you start feeding today. I'd wait until February to start. Azaleas, Camellia, Rhododendron Food, is the traditional fertilizer.
However, because of the sandy soil, I'd use Osmocote instead. Osmocote is a time-release fertilizer that releases some nutrients with every watering. It is applied twice a year, but in your case, I'd use it three times; in late February, early March, May, and August.