Modern cut flower carnation types are increasingly bred for disease resistance, so choice of variety has a part to play in the ultimate health of the plant. All members of the carnation family need a well-ventilated situation with free circulation of air, sharp drainage and plenty of sun. They don't like wet feet or damp, humid conditions. Cater to these needs and they will be well prepared to combat attack.
Dianthus and border carnations are generally easy-care plants and control of pests and disease poses few problems for home gardeners. The ease with which old garden pinks escape and multiply in the wild is proof of their tough constitution. Carnations grown in greenhouse conditions need more attention and should be checked regularly for evidence of pests, particularly on the undersides of leaves and in the tiny crevices around developing buds.
If you decide intervention is necessary, chemical insecticides act in two different ways. Systemic pesticides, usually applied by spraying leaves and stems, are absorbed by the plant and carried by the sap to all its living parts. Any insects sucking the sap will be killed. Systemic insecticides protect the plant for a specified length of time.
Contact insecticides are exactly that. They kill any insect they hit and are applied as sprays, dusts or smokes. Many of these contain Malathion, which is dangerous to humans. Less potentially damaging organic insecticides, containing pyrethrum, are available. Organic methods should also be considered. Care must be taken at all times when dealing with chemical pesticides or fungicides. Sprays are easily carried by wind, so choose calm weather and do any necessary spraying in the evening when bees are less active.
Just as bacteria that attack humans have developed resistance to antibiotics, there is a similar risk of bugs developing resistance to chemical killers. Alternating between the types of chemicals you use-or between organic methods-will help to prevent this.
The nectar of dianthus attracts ants and when the plants start to flower, they climb into the open flowers for a feast. If the petals of your carnations fall out and the flower collapses, blame ants-they could be nibbling on the center tips of the petals. Dusts containing pyrethrum or pirimiphos methyl will control them effectively.
All gardeners are familiar with greenfly, those tiny insects that swarm onto plants at the height of the growing season and greedily suck the sap, particularly from juicy young shoots, leaving sticky honeydew in their wake. They multiply at an alarming rate and like other aphids they look unsightly, their sap-sucking habits debilitate plants and, most seriously, they often transmit virus from diseased plants to healthy ones. Virus disease becomes apparent in the form of yellow streaks or spots on the foliage.
If you grow only a few plants it may be possible to control these annoying insects by physically squashing them on a continuing basis. Systemic pesticides are effective, but spraying will need to be repeated at regular intervals over summer. Some gardeners report success controlling aphids with an organic "tea" made from rhubarb leaves. Garlic planted beside dianthus has been found to deter some pests, including carnation fly.
Birds will chew on carnations, especially in spring. The plants can be protected by stringing black thread above them. In rabbit-prone areas, wire-netting fencing, set well into the earth, is the best protection. Mice can sometimes be a problem in winter, eating plants in greenhouses. Use mousetraps or the resident cat, so long as it is trained to treat flowerpots with due respect.
This fly can be a problem on carnations and pinks grown outdoors. It lays its eggs on the foliage of the carnation. Once hatched, the maggots feed on tissues between the upper and lower skins of the leaves and create whitish tunnels in their progress towards the main growth, and eventually the central stem of the plant-hence the name "leafminer." If the tracks are visible on leaves, cut the leaves off and preferably burn them. Otherwise, try to extract the maggot before it can pupate in midsummer, ready to start the next life cycle.
Handpicking is effective where leaves or plants are already mined. Maldison or pirimiphos methyl based sprays can help where damage is not too advanced. Planting garlic alongside plants may be an effective deterrent.
There is a range of caterpillars with varying habits. Some feed by day, others by night-when they can be sought by flashlight, picked off and squashed. Some are harder to find, secreting themselves inside the calyxes of carnations and pinks while they feed on flowers, born or unborn.
The tortrix moth caterpillar is cunning. It's almost as if nature knew it needed protection against modern spray chemicals, for the tortrix moth caterpillar has the ability to spin a fine web and roll the growing tips of the plant around itself as a shield, meanwhile growing plump and long (3/4 in /2cm) on carnation foliage. The moth is tiny and bright orange in color, an unexpected metamorphosis from the rather large caterpillar.
For control, use one of the following insecticides: maldison, acephate, carbaryl, pirimiphos methyl, permethrin or bifenthrin.
Another of the sap sucking variety, this insect is most visible by the frothy deposit it exudes as a sun umbrella to shield the bug while it carries on its nefarious business. Usually seen in the junction between stem and leaf, this bug should be treated as for aphids if it becomes a real problem.
Not usually regarded as any kind of pest, earthworms nevertheless are a problem in pot culture where they have a restricted run. Their prolific casts tend to clog the soil and impede drainage. To remove them, first allow the soil to dry out, then irrigate the pot with water to which lime has been added. This causes the worms to come to the surface, where they can be picked off.
Nasty creepy-crawlies these-they chew buds and petals-but you can deal with them effectively and in an environmentally friendly way. Loosely fill pots with moss, straw, grass or even crumpled newspaper and set them out on their sides among the carnation plants for the earwigs to shelter in at night. Dispose of the critters on a daily basis.
It is probably the most threatening enemy of the genus and is especially damaging in greenhouses, where it spreads quickly if unchecked. Yellow or brownish, rather than red, this spider is so small it can be seen only with the naked eye once it has formed colonies, when it looks like a type of rust on the undersides of leaves. It attacks the leaves, puncturing the surface to suck out the contents.
The first indication of the mite's presence is usually the appearance of pale blotches on the upper leaf surface. The leaves lose their waxy covering, become dull, turn yellow and die. Once a plant is depleted of moisture, these ingenious invaders quickly move on. Unable to fly, they crawl to the top of the dying plant and start spinning furiously. The aerial "ropes" swing them across to the next healthy plant and they continue on their destructive passage.
Red spider mites thrive in hot, dry conditions. Shading the greenhouse to reduce temperatures and raising the level of humidity are two ways of creating conditions less conducive to the mites' prolific life cycle. Maximum ventilation is necessary, as humidity can encourage the growth of fungal infections. Simultaneously, treat the plants with acaricides, spraying the foliage and watering the soil in the pot. Repeat the spraying every six or seven days to destroy the next cycle of mites. The following ingredients are all effective acaricides: bifenthrin, propargite, dienochlor, abamectin, dicofol.
Dianthus foliage and stems are not particularly attractive to these slimy pests but where they are a problem, tempt them to their deaths with a saucer of stale beer. Alternatively, set out snail bait, protecting birds and pets by placing the bait in short pieces of broken drainpipe, small jars on their sides, or similar coverings.
Like aphids, these tiny winged insects are sapsuckers. There are several varieties, but it is the onion thrip that attacks the unopened buds of carnations, leaving the flower to emerge with ugly, pale blotches where the color pigment has been sucked out.
Thrips breed outdoors in summer and in greenhouses during winter, laying their eggs on the calyx; the developing nymphs cause the damage. Thrips are especially prevalent during a hot dry period in summer. Either derris powder (if available), carefully applied to reach into folds in leaf and bud, or a systemic insecticide, will control this pest.
These pests are a threat to all plants and are especially prevalent in areas of grassland. About 1 in (2.5 cm) long and orange-yellow in color, they live in the soil for three to five years where they pupate, eventually appearing as a beetle. They munch plant roots-even burrowing up into the stems of carnations and dianthus-and cause death. An old-fashioned remedy is to bury pieces of potato or carrot just below the soil surface and mark them with sticks. The vegetables become infested with the worms and are easily removed and destroyed.
Harmful fungi, bacteria and viruses cause disease. Home gardeners who grow their dianthus outdoors are unlikely to be seriously troubled by disease. Plants grown in greenhouses are more vulnerable and the results of continuing research on carnation diseases are of much more importance to commercial growers of perpetual-flowering carnations. In the early years of last century the dread disease of rust forced some carnation growers out of production. With increased knowledge and improved growing conditions, this is rarely a problem now.
The best defense in the greenhouse is always good hygiene combined with correct cultural conditions. Always check new plants before you buy them. Introducing diseased plants to healthy stock is a surefire method of spreading disease. The whole greenhouse should be disinfected once a year preferably in the fall. Use an antibacterial and anti-fungal disinfectant at the recommended strength and wash down the glass walls, all the shelves and pots, and don't forget to also disinfect the tools you use.