Insect repelling and insect resistant plants.
Plants that don’t get ‘buggy’ have plenty of advantages in landscaping: in the first place, they always look good, and in the second, they save on the time, money and risk involved in regular spray programs.
Low maintenance, low risk and low water requirements make for a powerfully attractive combination.
There’s plenty of scope for a clever and creative landscape using environmentally appropriate material. Plants that are able to thrive in a given environment are healthy, and healthy plants tend to have natural defenses against insects and diseases.
Some plants have very strong defenses against insects and may have insect repellent or insecticidal properties. Many of them can be used to make herbal infusions that can be sprayed onto other, less fortunate plants. Some environmentally friendly, low-risk insecticides and insect repellents are easy to make. They can be very effective. Given that many plant diseases are transmitted by insects, there is a spin-off in the form of reduced disease transmission.
Insecticidal herbal infusions are old folk remedies, but they are becoming increasingly accepted in mainstream agriculture and horticulture, so that more and more commercial preparations based on this ‘folk knowledge’ are becoming available.
As pest resistance to chemicals and possible health risks eliminate more and more chemical preparations from the market, the use of plants that don’t require spraying and the use of organic preparations for those that do become more and more important in both agriculture and ornamental horticulture.
Insect resistant indigenous plants
Most indigenous plants are reasonably tolerant and will deal with occasional insect outbreaks, but some have excellent defenses against insect attack. A few examples based on our experience with growing a wide range of indigenous plants are listed below.
Acmadenias such as Acmadenia heterophylla are genuinely attractive plants with pretty pink flowers.
Despite their delicate appearance, they are very resistant to insect attack. Coleonemas (the ever-popular confetti bushes) fall into a similar category – delicate-looking, but unpalatable for most insects.
Buchus (Agathosma spp) are not nearly as difficult to grow as many people think. Their leaves are covered with aromatic oil glands, and insects are repelled by the strong aroma.
This group of plants can provide a good component of insect repelling infusions that can be used to protect more delicate plants such as roses. Fortunately, people are very attracted to the smell of Buchus and they have unusual flowers into the bargain.
The Natal laburnum, Calpurnia aurea, can easily be pruned into a standard or lollipop for formal gardens. It contains a natural insecticide in its sap. Beetles, caterpillars and other pests leave it religiously alone.
Diospyros spp such as D.dichrophylla are extremely robust. These small trees not only tolerate a wide range of conditions, but they are very pest resistant.
Restios include a wide variety of hardy genera such as Chondropetalum, Thamnochortus, Elegia, and Rhodocoma.
They are very effective as landscaping plants, providing effects that range from the structural to the tropical and informal. They are rarely, if ever attacked by insects.
Free-flowering Gnidia squarrosa and other species within the genus such as G. oppositifolia and G. pinifolia, although not aromatic, are shunned by insects and can be relied on to look good throughout the year.
Tarchonanthus camphoratus – with correct pruning – a very hardy little tree Tarchonanthus camphoratus.
Pelargonim spp tend to be very insect resistant. Rose scented pelargoniums such as P. capitatum are recognised as good companion plants for more susceptible species, especially in repelling aphids.
Nutmeg scented pelargoniums (P. fragrans) and the medicinal P. reniforme thrive in nursery conditions without needing insecticide treatments. Pelargoniums grow best in sun or very light shade, and can be prone to whitefly if planted in deep shade.
As with many of the strongly aromatic plants, Eriocephalus africanus and other Wild Rosemaries are seldom, if ever, attacked by insects. They are highly ornamental, and the silver foliage can be used to good effect in creating striking contrasts. Aromatic plants are good components of insect repelling teas, and this one is no exception.
Tarchonanthus camphoratus (Wild Camphor), like most aromatic plants, is generally avoided by insects. I have never had to spray them in the nursery. They have a very widespread distribution in South Africa and are extremely hardy and adaptable.
Tulbaghia violacea has insect repelling properties. Although onion thrips and weevils occasionally nibble on them, they are generally pest resistant, and won’t require spraying. The are often regarded as good companion plants.
Watsonia spp such as W. borbonica can be either deciduous or evergreen. They are seldom preyed on by insects and are very drought hardy into the bargain.
Pest prone indigenous plants
Although most indigenous plants are seldom so prone to pests that they will actually die from attacks, there are a few that are regrettably susceptible to attack. It helps to know what to expect, as many of these are also very showy. If we are alert at the appropriate times of the year, we can be prepared to deal with the problems as they arise.
Most of these will recover nicely once they are established, so a little care during the early stages is often all that’s needed. A few examples:
Cussonia thyrsiflora can grow upright, as a shrub or even indoors. It tolerates sun or shade. Erythrina spp are not only attractive to people, but sadly to a wide range of pests including red spider mite, borer beetles that can cause die-back, red scale and pernicious scale. Once they are established in the landscape, they become less prone to attack, but one needs to be vigilant.
Gardenia spp are very prone to mealy bug and scale, and the resultant sooty mold can be the ruin of an otherwise attractive plant.Cussonia spp with the exception of C. thyrsiflora are very prone to attack by psylla, which cause ugly blisters on the leaves.
Rhus lucida provides shelter for a colourful beetle. Rhus spp though otherwise very hardy are often attacked by a type of caterpillar that feeds socially and strips one branch after another of its leaves. Although established plants will recover easily, young plants can be defoliated to the point where survival is difficult.
Cyrtanthus elatus and many other bulb species are very attractive to lily borer caterpillars in spring and autumn. They hatch on the leaves and work downwards into the bulb, killing the plant. If they are caught soon after hatching, they can be hand-picked or sprayed. Vigilance is the key. Some bulb species that are seldom if ever attacked by this pest include Watsonia spp, Amaryllis belladonna, and that hardiest of Chinkerinchee species Ornithogalum longibracteatum otherwise known as the ‘pregnant fairy’.