This week: an extract from my book, the Vertical Veg Guide to Container Gardening. The book covers all aspects of container gardening and this extract looks at how to grow healthy plants in containers and how to navigate the joys and challenges of growing alongside the birds, insects and molluscs that frequent our balconies and patios.

I grew up at a time when wasps were evil (or so we were told) and where nearly every garden shed had a padlocked box full of herbicides and pesticides. I’m grateful to all those forward thinking farmers and gardeners who have helped me to see the world through different eyes.

Growing in Harmony with Nature

Growing can seem easy and straightforward when plants are healthy and happy. However, when they get sick, are eaten by molluscs or just don’t grow well, it can feel harder and more mysterious. If problems are serious, it’s easy to get demoralised.

When something goes wrong, it’s important to remember that all growers experience ups and downs and occasional disasters (even if gardening programmes rarely feature them). It’s a normal part of growing, and unhealthy plants are not a reflection of your ability to grow. If you can keep a positive attitude, and an inquiring mind, gardening problems also offer some of the best opportunities to learn.

When a plant looks unhappy, the first step is to try to identify the cause. Sometimes it’s obvious: caterpillars munching on nasturtiums, strong winds snapping stems or plants drooping due to lack of water. But when plants just aren’t thriving, a little detective work is often needed. Some cases are harder to solve than others, but hopefully the information below will help.

What is a ‘pest’ and when are they a problem?

All life forms have a role to play in a healthy ecosystem. Slugs and snails recycle nutrients in the soil, and are food for thrushes, hedgehogs and ground beetles. Aphids are food for ladybirds and are fed by sparrows to their chicks. Hoverflies are an important pollinator and their larvae eat aphids voraciously. Even the maligned wasp hunts caterpillars and helps with pollination.

Is ‘pests’ the right word to call this precious life that helps keep everything in balance?

Small populations of slugs or aphids rarely do much damage. It’s only in large, uncontrolled numbers that they can. The challenge is that many container gardens are too small to have a full, balanced ecosystem to keep them in check naturally. Most balconies don’t have hedgehogs, ducks or ground beetles to eat slugs. Without a predator, slugs multiply uncontrolled and then they can make growing difficult.

The upshot is that you often need to be the predator and find and remove slugs and other plant eaters from your container garden.

Cultivating a different mindset can help in our relationship with slugs and other animals. In the west we are taught that ‘pests’ are bad. But early farmers in some indigenous cultures had a different view. They valued all life and were happy if a proportion of their crops were eaten by other animals. They saw it is a way of giving back to the wider web of life. Making this shift in our own thinking – and realising that our growing can support other animals – makes it more rewarding. And less demoralising if plants are eaten.

Six ways to grow healthy plants in harmony with other life

The secret to a healthy container garden with minimal problems is prevention. There are no magic solutions, but the most important is to promote plant health and reduce plant stress. I describe various prevention techniques below and, taken together, they will contribute towards a healthy, lush and vibrant container garden.

1. Promote plant health, reduce plant stress

Like us, plants have a range of natural defences that they deploy against attack from predators and disease. They produce chemicals to make them taste unpleasant to insects, release smells to attract aphid-eating bugs and some can even send signals through their roots to warn other plants about encroaching predators.

Also, like us, their ability to resist disease and insect attack depends largely on how healthy they are. If stressed or weak, they are less able to respond effectively. The underlying reason for aphids or disease in container gardens is normally not ‘a plague of aphids’ but stress and poor plant health. Aphids are often a symptom of a problem, rather than the problem itself.

In our culture, it is common to focus attention on the ‘pest’. But it’s often more helpful to turn this on its head and think about the health of the plant and particularly to try to reduce any stress.

Life can be particularly stressful for urban pot plants. Because of the confines of a container, they are prone to a shortage of food and water, and container gardens are often shady or windy. Minimising the sources of stress can have a major impact in helping to grow strong and healthy plants.

Common sources of stress

Here are some common sources of stress for plants in containers (in italics), and how you can minimise these.

• Lack of water/irregular watering/too much water. Daily routine watering routine, mulching, large pots, containers with reservoirs.
• Lack of minerals, trace elements and soil life. Use a good-quality compost, a range of natural, high-quality feeds, including some containing microbial life like worm compost.
• Too hot. Use water reservoirs, shade netting, select appropriate plants. Liquid seaweed can help plants recover from stressful heat events. 
• Too cold. Avoid putting plants outside too early in the season, harden off and protect if needed.
• Persistent or strong winds. Tie in taller plants. Use windbreaks or cloches. See Challenges chapter for more information.
• Moving plants/ transplanting. Handle plants and roots with care. Water well after moving. Choose an overcast, still day or shelter from the sun and wind.
• Too little sun/light. Select crops according to the sun available. In less sunny places, try to reflect as much sun as you can.

How to recognise healthy plants

Try to become more familiar with what healthy plants look like. Signs of good health include strong, steady growth, a sturdy stem and dark green leaves (the leaf colour will obviously also depend on the variety). They also tend to have more shiny and glossy leaves, due to a surplus of fat stored in their leaves, in what is known as waxy cuticle.

The healthier plants are, the wider range of pests they are resistant to. John Kempf has done some fascinating work in this area and developed the ‘plant health pyramid’ to illustrate how plants become increasingly resistant to pests as their needs are met.

Bright green, glossy leaves are a good sign of strong, healthy plants - photo by my friend, © Clare Bowes

2. Observe and remove

However healthy your plants are, they will still sometimes be eaten by slugs, insects and other creatures. A benefit of growing on your doorstep is that it is quick and easy to keep a close eye on this. Spotting and dealing with infestations early is the best and easiest way to manage them. Daily watering offers the ideal time to look out for signs of plants being eaten or in poor health – and to look for the culprits.

Once spotted, the most effective way to deal with slugs, snails and most insects in small spaces is to pick them off. They can then be fed to the birds (or given to someone with chickens), released in the local park or squished (and added to the worm bin). It’s sometimes beneficial to leave a few aphids or caterpillars on the plants as these will help attract ladybirds and other beneficial insects to your garden. However, you can’t always rely on ladybirds turning up in urban areas so it is important that you also take on the role of predator and stop the aphid population getting out of hand if necessary.

Always try to identify any insect before removing it (Google or an insect-identification app like ‘Seek’ make this much easier). Otherwise, it’s easy to unwittingly remove hoverfly larvae or ladybird larvae, two of the best aphid munchers out there!

Not everyone finds it easy to pick off slugs or caterpillars at first, but it’s worth trying to get used to it. It might seem cruel but it’s far more controlled and discriminate than killing with chemicals. It’s also the most effective way to control slugs and most plant-eating insects.

Not all slugs are equal

Not all slugs eat fresh green leaves and some cause more damage than others. One to watch out for is the Spanish slug, a large dark brown beast that is becoming more widespread across Europe. It makes short work of baby plants, and can even be heard munching them!

Other slugs eat predominantly rotting vegetation. They do little damage in the container garden and can usually be left alone to get on with things. A common variety is the Green Cellar Slug, often found in wormeries. The John Innes Institute has a good online guide with pictures of the different slugs.

The common Green Cellar Slug eats mostly decaying vegetation and doesn’t harm living plants. It’s often found in wormeries.

3. Grow a diverse mix of crops

A diverse mix of plants makes it harder for insects and animals to see or smell what they want. And, if they do find your container garden, they’ll often only be interested in one or two plants, leaving the others undamaged. Pigeons, for example, eat mostly kale and other brassicas but won’t touch tomatoes, courgettes and most other fruit and vegetables (and, in my experience, they also aren’t very good at finding brassicas if they are hidden amongst other plants).

A diverse mix of plants makes it harder for pests to find what they want and reduces the risk.

A diversity of crops also increases the chance that some will do well in our increasingly unpredictable weather. For example, a cold, wet August and September will not suit tomatoes and chillies well, but establishing winter crops like kale and chard will love it.

The problems with chemical control

A wide range of chemicals and pesticides are sold to ‘manage’ pests and disease. Unfortunately, most also indiscriminately kill other lifeforms, add poisonous chemicals to the food chain and damage the environment. Even ‘organic’ slug pellets can harm microbial soil life which, in turn, weakens the plant and its ability to resist attack. It’s a vicious circle. I believe that pesticides, even homemade organic ones, are never needed in the container garden.

4. Use barriers

Barriers like netting and cloches protect plants from hungry wildlife. Fine mesh insect netting is effective at keeping out aphids, butterflies, leaf miner and other insects, while larger mesh netting keeps out birds and heavier weight chicken wire does the same for squirrels and animals with sharp teeth.

Netting is effective but it is also unsightly in the container garden. Securing it is time consuming and a bit of a faff. It’s also another thing to buy and store. However, it’s useful as a last resort and, for bad bird or insect problems, it’s sometimes the only practical solution. In our home, without netting, we’d never get any blueberries to eat (the local blackbirds love them).

To support netting over leafy crops, make hoops over pots with flexible plastic plumbing pipe, old metal coat hangers or plastic hula hoops. Then drape the netting over and tie it round the base of the pot with string. On fruit bushes, it’s often possible to drape the netting directly over the plant and secure it with clothes pegs. Be aware that some birds are very determined and will find a way through surprisingly small gaps.

5. Grow disease-resistant varieties

You can buy specially bred crop varieties that offer varying degrees of resistance against common diseases like blight. Resistant varieties don’t offer total protection, but they do help to delay the onset of the disease or reduce its impact. If your tomatoes or potatoes are decimated by blight every year, a resistant tomato like Crimson Crush, or potato like Sarpo is well worth a go.

6. Edit out difficult crops

If a crop is difficult to grow every year because it gets attacked or diseased, sometimes it’s best to listen to nature and just stop growing it, at least for a season or two. I gave up growing New Zealand oca because the tubers got eaten by slugs or vine weevils every year, making it rather disheartening.

The secret to healthy plants in containers is simply ensuring they get enough sun, water and food and are in good quality compost in a large enough container. Photo © Clare Bowes / Vertical Veg