UNITED STATES—Plants are so much more intelligent than they get credit for. Many use color, fragrance and flavorful nectar to get insects and animals to disperse their pollen for them. Some provide fruit for animals that inadvertently take and disperse their seeds. Others use barbs or sticky substances to attach their seeds to unknowing animals that take them away. Plants have all sorts of techniques for exploiting those who are more animated than they are. After all, immobility has certain disadvantages.
Because plants can not get away from the animals and insects that eat them, many have developed techniques for being unappealing. Plants that live in deserts where edible foliage is relatively scarce are famous for their nasty thorns and spines, like those of cacti and agave. Hellebore and poinsettia have caustic sap, which makes them unpalatable. The naturally aromatic foliage of many edible herbs, like rosemary and lavender, is actually intended to repel grazing animals with sensitive noses.
Some plants unfortunately rely on toxicity for protection. Many plants are only partially toxic. For example, apples are intended to be eaten safely by animals that disperse the seeds within, but their seeds are toxic enough to avoid getting eaten by rodents after dispersal. Potato, tomato, rhubarb, asparagus and elderberry plants all produce edible fruits or vegetables, but also have poisonous parts. Some edible fruits and vegetables, like grapes and onions, are edible to humans, but toxic to dogs.
Foxglove, angels’ trumpet, morning glory, yew, rhododendron, azalea, oleander and castor bean are some of the more notably toxic plants often found in home gardens. Wisteria, holly and ivy produce toxic seeds and fruits. Dieffenbachia is a popular, but very toxic houseplant. Although mostly safe, toxic plants can be a problem where young children might put things into their mouths, or where puppies are in that chewing phase.
As long as freeways have been getting landscaped, oleanders have been contributing their profuse white, pink and red bloom. Heat, exposure and lack of moisture do not seem to bother them. They have become less common recently only because of new diseases that had never before been problematic. The diseases do not necessarily kill all oleanders everywhere, but are serious problems where the nurseries that grow most oleander are located.
The largest oleanders can get more than fifteen feet tall, and can be pruned up as small trees with multiple trunks. Oleander trees with single trunks almost never stand up straight, and do not want to give up their stakes. Because flower clusters develop at the ends of new growth, frequent exterior pruning or shearing inhibits bloom. Dwarf cultivars that are naturally proportionate to their space will bloom better than larger types that need to be pruned for confinement.
Oleander flowers are about an inch or two wide, with five petals, although some have ruffly ‘double’ flowers. Unfortunately, double flowers tend to hang on as they deteriorate after bloom. Some oleanders are slightly fragrant. The name ‘oleander’ is derived from the similarity of their leaves to those of olive trees (‘Olea’), although oleander leaves can get three times as long.