Why do plants get sick? The simple answer is for lots of reasons, many of them similar to the reasons why we humans do. Take, for example, poor diet. Just as it’s one of the root causes of disease, poor growth and reduced life expectancy in humans, so it is with plants.
That malnutrition could be the result of a nutrient-poor soil or of overcrowding, where there simply isn’t enough food to sustain healthy growth. Alternatively, it could be because of certain growing conditions such as drought, a waterlogged soil, a compressed soil, or a very acidic or alkaline soil making it difficult for a plant’s roots to easily access key nutrients locked in the ground. Or it could be because of a polluted soil that’s been contaminated by toxic chemicals.
Either way, the result is stunted and/or distorted, discoloured growth, reduced flowering and/or fruiting and a general lack of vigour that then leaves it vulnerable to pests and disease.
Just like humans different plants also respond in different ways to the environments they find themselves in. Ericaceous plants such as azaleas, rhododendrons, camellias and some species of magnolias, for example, naturally thrive in a damp but well-drained acidic soil with a pH lower than 7. But plant them in an alkaline soil and they’ll quickly develop a condition known as lime-induced chlorosis, where their leaves turn yellow. This yellowing of the leaves is caused by the fact that the alkaline soil in which they’re growing interferes with an ericaceous plant’s ability to absorb key plant elements, resulting in an iron deficiency that’s the horticultural version of anaemia.
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But that’s not all. In exactly the same way that nutritional deficiencies in the human diet have a cascade effect in terms of our overall health, the lack of chlorophyll (the pigment that normally makes leaves appear green) in the leaves of a chlorotic plant has other much more serious consequences, chief of which is its inability to photosynthesise. Robbed of that all-important ability to absorb energy from light, it typically weakens and dies.
Similarly, other plants can become ill as a result of cumulative stress caused by inappropriate growing conditions ill-suited to the particular species. Plant a heat-loving species of decorative shrubs and small trees such as Californian lilac (ceanothus) that’s native to much warmer climes into a cold, exposed Irish garden and it will suffer to the point where it grows weak and eventually dies.
In the same way if you stick a shade-loving, spring-flowering woodland species into a hot, dry border it won’t thank you for it. Nor will a drought-tolerant species of succulent or cactus that’s evolved over thousands of years to cope with extreme heat and a dry soil, but instead finds itself presented with a cold, wet, poorly-drained site. All of which is why seasoned gardeners always do their very best to follow that age-old mantra of “the right plant for the right place”.
Even then container-grown plants in particular can suffer from stress and ill health no matter how well situated or appropriately selected they are for a garden’s growing conditions. This is because their vulnerable root systems are far more exposed to temperature extremes than if grown in the ground, while they’re also more vulnerable to the effects of drought, wind and even waterlogging.
Unless the compost in which they’re growing is regularly top-dressed and occasionally replaced, container-grown plants are also far more likely to eventually suffer from malnutrition once the plant has used up all the available nutrients contained within the compost. Alternatively, they can suffer from what’s known as “salt-burn” as a result of a gradual accumulation of fertiliser residues in the compost. Add to this that their ever-questing root systems will eventually reach the sides of their containers, causing them to become what’s known as “pot-bound” (the horticultural equivalent of us humans walking around in a pair of shoes several sizes too small for us) and it’s no small wonder that they’re often unhappy.
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In this weakened, stressed malnourished, state these plants are also less able to fight off many common species of sap-sucking and leaf-boring plant pests. This is why it’s not unusual to see a bad infestation of aphids thriving on a neglected container-grown plant that’s been underwatered, or one that has become pot-bound.
Just like humans a plant’s age also affects its overall health and ability to tolerate stressful conditions. Young seedlings, for example, will quickly succumb to common plant stressors such as a shortage of water or sharp extremes of temperature while their mature equivalent will not.
If you’ve ever grown your own summer bedding plants then you will have experienced this at the freshly-germinated seedling stage – when they must be carefully cosseted and protected against low temperatures and chilly drafts if they are to survive – whereas later in the growing season the mature plants are a great deal more resilient.
Likewise, young soft plants raised under the protective cover of a polytunnel or glasshouse and then abruptly placed outdoors to face the rigours of the Irish weather will also, quite understandably, display common symptoms of stress, including dropping their leaves (an extreme way to conserve water and reduce the pressure on their immature root systems), curling of leaves (another way to conserve moisture) and dropping of flowers and flower buds (ditto).
At the opposite end of the scale very mature plants reaching the natural end of their lifespan are also more vulnerable to pests and diseases.
Physical injuries to a plant as a result of storm damage, poor pruning techniques or just badly-timed pruning can all leave it far more vulnerable to pests and diseases. Prune a pear tree in autumn or winter, for example, and you greatly increase the chances of it falling prey to the destructive fungal disease known as “silver leaf”. But if you prune it in on a dry, bright summer’s day when there are far fewer fungal spores about and the pruning wound is more likely to quickly heal over, then you greatly reduce the risk.
Likewise even relatively small injuries to a plant as a result of wind, hail or frost damage, clumsy strimming, grazing by animals, or even just regular chafing of an overly tight tree-tie can allow entry to pests and diseases from which its tough bark or outer skin of plant cells would otherwise protect it, much like a human suffering blood poisoning as a result of accidentally walking on a rusty nail.
In short, plants are surprisingly like people when it comes to many of the things that make them ill. But show them the same consideration that you would a friend or loved one and they’re much more likely to stay healthy and happy.
This week in the garden
- The first killing frost of autumn arrived in many Irish gardens last week, signalling the arrival of late autumn and the end of another growing season.
- Now is the time to start planting and planning for next year by planting bulbs as well as many kinds of trees, shrubs, perennials and climbers.
- Clear out summer containers, place frost-tender perennials and shrubs under cover, refresh the compost and replant with a mix of hardy bedding, evergreen shrubs and spring-flowering bulbs.
- If you sowed seed of hardy annuals back in late summer-early August then it’s important to get the young plants in the ground as soon as you can so that they can get well-established before the arrival of winter proper.
- Always prepare the ground well before planting by removing weeds and working some well-rotted garden compost or manure into the soil.
- Yung module-raised transplants will also benefit from having their root ball soaked in a solution of liquid seaweed just before planting.
Dates for your diary
- October 26th (8pm), Northridge House, St Luke’s, Castle Road, Mahon T12H970 Cork. “Spring Bulbs”, a lecture by Richard Hobbs of Witton Lane Seeds, Norwich, on behalf of Cork Alpine Hardy Plant Society, all welcome, admission €10
- From Sunday, October 29th until Sunday, November 12th National Botanic Gardens, Glasnevin, Dublin. “Crainn na hÉireann”, an exhibition of over 60 paintings produced over the last two years by members of the Irish Society of Botanical Artists as a celebration of the seasonal beauty of Ireland’s native trees, see irishbotanicalartists.ie
