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Gardening: Become a plant doctor

Iam often asked to explain why plants are sick or dying.

Leaf condition is the prime indicator of a plant's health, so keep your eyes open for any signs of troublephotograph: shutterstock
Leaf condition is the prime indicator of a plant's health, so keep your eyes open for any signs of troublephotograph: shutterstock

To diagnose problems, you need to know if a plant suddenly keeled over and died or whether death was slow and steady. One of the joys of gardening is watching plants as they grow, and alarm bells should start ringing if they begin to look a little unusual. Once you know what's wrong, you may be able to save it.

Plants only thrive in the right conditions - in good soil, with the necessary food and water, and protection from the wind. And they are constantly besieged by other living organisms - animals, insects, fungi and viruses.

Leaf freshness and colour is the best indicator of a plant's health and signs of suffering include discoloration, contortion, nibbling or withering. The plant should also spread and grow at the correct rate for its species or variety and there shouldn't be dieback or bare stems.

This week, I'll look at growing conditions and will turn the spotlight on a plant's assailants next time, but I must stress that this is a complex subject. You can easily tell if a slug has consumed a seedling, but identifying a fungus or understanding the biochemical process leading to a plant's disorder is beyond lesser mortals like me.

Different plants need different types of soil. Although some require acidic soil, with a low pH, most need a more alkaline ground, pH 6 or higher. Ericaceous rhododendrons and camellias do poorly in more alkaline soil because they can't access the larger amount of iron that's available in acidic ground. With too little iron, young leaves may be bleached or yellowing and have pronounced dark veining. They may have fewer flowers and hydrangeas even produce pink blooms rather than the more usual blue ones.

A low pH, though, adversely affects most garden plants. The leaves are often yellowish and plants grow poorly and are more vulnerable to attack from fungi and viruses. Test the soil's pH - you'll quickly see whether it suits your plants.

The three key elements in the soil - phosphorus, nitrogen and potassium - are more readily available in ground with a pH higher than 6. Phosphorus is required for good root growth, nitrogen for stem and leaf growth, and potassium at the flowering and fruiting stage.

If a plant lacks phosphorus, it develops a poor root system. Its leaves are smaller than you'd expect, they may be red or purple or may simply drop without discolouring. Phosphorus can also be leached away from light, sandy and very free-draining soil. The solution is to bulk up the soil with compost and other organic material.

However, soil is much more likely to lack nitrogen than phosphorus. Older leaves may be tinged red or purple, but they usually turn yellow. When you identify these symptoms, top dress the soil with pelleted chicken manure and consider growing nitrogen-fixing plants such as peas, beans or lupins.

Like phosphorus, potassium is more freely available in alkaline soils but less so in poor, thin ground. Look out for brown scorching on the tips of the leaves or brown spotting on the undersides of the leaves.

But problems often lie with the elements -wind, rain and frost. Scorched and wilting leaves are common after a frost or if a young plant hasn't been hardened off. Wind can also cause this, especially in exposed places, and wind rock damages the roots of young or tall, thin plants, causing poor and stunted growth.

Look out for signs of too much or too little water, too. Water feeds plants by dissolving vital nutrients in the soil and without this, leaves brown, wilt and drop. Large-leafed plants succumb quickly to drought.

At the other extreme, waterlogged ground washes away nutrients, leading to yellowing leaves, rotting roots and drowned plants. You can have too much of a good thing.

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