Turn your rooftop into a garden of chrysanthemums

Most city rooftops spend the year as grey concrete. Washing lines, a water tank and the fierce midday sun: little else happens up there. Yet when winter comes, the same roof can fill with yellow, white, red and pink, if a few pots of chrysanthemums have been planted. Known for their sweet scent and bright colours, chrysanthemums can, with the right care and a little planning, turn an ordinary roof into a beautiful and restful garden.

The chrysanthemum is a winter flower. According to the Agriculture Information Service, it prefers relatively cool weather and a sunny position, and in Bangladesh winter is the best season to grow it. Gardeners love it for many reasons: the range of colours, the long flowering, the way it adapts to pots, and the good results it gives for modest care. For anyone starting a rooftop garden, it makes a fine first choice.

In Dhaka and the country's other big cities, green space is shrinking fast. Among the crowd of buildings, bare earth is hard to find, and the roof has become the city dweller's biggest open space. A roof covered in plants keeps the sun from beating straight down on the concrete, so the rooms below stay a little cooler. Birds and butterflies return, and the family gains a place to sit in the open air.

The government is encouraging rooftop gardens too. In June 2023 the Local Government Minister announced that house owners in all city corporations and municipalities would receive a 10 per cent rebate on holding tax for a rooftop garden. The stated aim was to lower urban temperatures and make the country greener. But a few pots are not enough to qualify: the garden has to be kept up properly, and care must be taken that standing water does not breed mosquitoes. For the detailed conditions, it is best to ask your own city corporation or municipality.

There are many flowers for a rooftop garden, but a few qualities set the chrysanthemum apart.

The first is colour. Chrysanthemums come in yellow, white, red, pink and more. The flower shapes vary too: some small and round like buttons, some with long curling petals, some as big as a ball. Plant a few varieties together and the roof comes alive.

The second is modest care. The plant grows vigorously and flowers well without much attention. Regular sun, measured water and occasional feeding are essentially all it asks.

The third is fresh air. In a well-known 1989 study by the US space agency NASA, several plants kept in sealed chambers absorbed harmful chemicals such as benzene and formaldehyde, and the florist's chrysanthemum was among the most effective. But those tests were done in small, closed chambers. Later researchers have pointed out that in ordinary homes, air exchange with the outdoors is the main way pollutants are removed, and a few potted plants make little difference. So chrysanthemums certainly make a roof greener and pleasanter, but they should not be seen as a substitute for an air purifier.

The fourth is medicinal use. Some chrysanthemum species are traditionally believed to have medicinal value as well as a soothing scent. In East Asia, and in China especially, tea brewed from dried chrysanthemum flowers is an old tradition. But flowers from garden plants treated with chemicals should not be eaten, and anyone thinking of using them to treat an illness should first ask a doctor.

The chrysanthemum's scientific name is Chrysanthemum morifolium. It is a bushy plant with lobed, faintly aromatic leaves. One of its particular habits is that it prepares to flower as the days grow shorter; botanists call it a short-day plant. That is why buds form in late autumn and early winter, as the nights lengthen, and the plants are covered in flowers in December and January. Bright lights left on over the plants at night can upset this natural cycle, which is worth remembering on a roof.

The name has a history of its own. Chrysanthemum comes from two Greek words meaning gold and flower: the golden flower. It was first cultivated in China more than a thousand years ago and later reached Japan, where it is so honoured that the emblem of the imperial family is a chrysanthemum. In Bengali it is known as chandramallika.

There are varieties suited to local growing. According to the Agriculture Information Service, the Bangladesh Agricultural Research Institute (BARI) has developed two, named BARI Chandramallika-1 and BARI Chandramallika-2. Nurseries and markets also sell many other local and imported varieties.

Timing matters. According to the Agriculture Information Service, October to November is the right time to plant out in the ground or in pots. In beds, depending on the variety, it recommends spacing of about 30 centimetres between rows and about 25 centimetres between plants.

New plants usually come not from seed but from cuttings or from shoots at the base. A piece 8 to 10 centimetres long, cut at a slant from a fresh, vigorous one-year-old stem and set in a bed or a bucket, will put out roots. And around February, when flowering ends, the plants are cut back to 15 to 20 centimetres above the soil; after a while new shoots, called suckers, come up from the cut base and can be separated and planted as new plants. Once you have a few good plants, you need not buy new ones in the years that follow.

Neglect the plants once flowering is over and next year's plants will be weak. The hot and rainy months are a hard time for the mother plants and their suckers. Because chrysanthemums cannot tolerate standing water, the soil in the pots must not stay muddy in the monsoon; if needed, raise the pots a little or move them under cover during days of steady rain. In this season the aim is to keep the plants alive, not to get flowers. When planting time comes round again at the end of autumn, the reward is strong, disease-free young plants.

For a rooftop gardener, the whole year can therefore be set out as a plain calendar:

• October to November: plant out in pots.
• About a month after planting: pinch out the tips so the plants grow bushy.
• Late November to December: buds form; disbud if wanted, and stake the stems.
• December to January: full bloom.
• February: once flowering ends, cut the plants back to 15 to 20 centimetres.
• After that: raise next season's plants from suckers and cuttings.

Follow this calendar and there is no need to start from scratch every winter; the garden keeps turning like a cycle of its own.

The whole process of growing chrysanthemums can be split into seven steps. Each is small, but skipping any of them reduces both the number and the quality of the flowers.

Step one: choose the right pot. Use a clay or ceramic pot to hold the soil. Clay lets air through and heats up less in the sun. Whatever the pot, drainage holes in the bottom are essential; without them water collects around the roots and rots them. A medium-sized pot is enough for one plant; a very small pot dries out too fast.

If you plan to keep many pots on a roof, think about weight. Clay pots are heavy; plastic ones are light and cheap, but plastic heats up quickly in strong sun and grows brittle within a few years. Many rooftop gardeners therefore mix the two, using clay for big plants and plastic for young ones.

Step two: prepare the soil. Mix loam with rotted cow dung or another organic manure. The soil should hold some moisture while still letting air move through it. On a roof, weight matters too, so it is best to avoid soil that is too heavy or clayey.

Step three: plant seedlings or seed. Get good-quality seedlings or seed; healthy, sturdy plants from a reliable nursery are the safest choice. Leave enough space between plants in pots or beds so that each gets light and air, and disease does not pass readily from one plant to the next.

Step four: water with care. Chrysanthemums dislike too much water. Water lightly only when the top of the soil feels dry to the touch. With more sun and wind on a roof, pots dry quickly in summer, but in winter they need less. Never let water stand in the saucer under a pot; it rots the roots and breeds mosquitoes.

Step five: feed. Give a liquid organic fertiliser once a month. As buds form and flowers open, a little extra potash and phosphate improves both the size and the number of flowers. Too much nitrogen gives plenty of leaves but few flowers.

Step six: sunlight. Keep the plants in at least 5 to 6 hours of direct sun a day. Chrysanthemums will not flower well in the part of a roof shaded by the water tank or a taller building next door.

Step seven: prune on time. Regularly cut away dried leaves and stems. Prune the plant back once every two months to encourage new growth. Fallen leaves and old flowers left in the pot invite fungal attack, so clearing them away is part of the care.

Experienced gardeners use two special techniques with chrysanthemums. The first is pinching. About a month after planting, the soft tip at the very top of the plant is pinched off between the fingers. Instead of growing tall, the plant then puts out several side shoots and becomes bushy, and because each shoot carries flowers, the total number of blooms goes up.

The second is disbudding. Gardeners who want large flowers keep the main bud at the tip of a stem and remove the small buds around it. All the plant's strength then goes into one flower, which grows much bigger. Those who prefer lots of small flowers can skip this step.

On large-flowered varieties, stems can bend under the weight of the blooms, and roofs are windy. A sturdy stick pushed into the pot beside the plant, with the stems tied loosely to it, keeps the plant upright and the flowers intact.

Among the chrysanthemum's common enemies are aphids, which suck sap from young tips and buds, and powdery mildew, a fungus that coats the leaves with what looks like white powder. A habit of checking both sides of the leaves catches problems early. Cutting away affected leaves and stems, leaving room for air between plants and watering at the base rather than wetting the leaves will keep many diseases away. If an attack is severe, it is safest to ask the local agriculture office, or the Krishi Call Centre on 16123, which treatment to use and how much.

People planting chrysanthemums for the first time often make a few mistakes. Knowing them in advance helps avoid them:

• Watering every day out of affection, until the roots rot.
• Keeping pots in the shade, so the plants grow tall and weak and flower poorly.
• Planting in a handsome container with no drainage hole.
• Giving too much nitrogen fertiliser in the hope of lush leaves.
• Forgetting to pinch the tips, so the plant stays a single tall stem.
• Leaving bright lights on over the plants all night in the budding season.

When buying plants from a nursery, check a few things: whether there are insects or white spots on the undersides of the leaves, whether the stem is firm, and whether healthy white roots show at the bottom of the pot. Yellow, drooping or overly leggy plants are best avoided. Starting with just a few plants and building a habit of care makes enlarging the garden the following year a far more confident undertaking.

Once the plants are growing well, it is time to arrange them. Where and how the pots are placed decides how the whole roof looks.

Make a pattern of colours. Use chrysanthemums of different colours to create a design in one corner of the roof: yellow in the middle ringed with white, perhaps, or waves of red and pink one after the other. Similar colours side by side feel calm to the eye; contrasting colours make the pattern stand out.

Arrange by height. Line up the pots according to the height of the plants: tall ones at the back, medium ones in the middle, short bushy ones in front. That way every flower can be seen and none is hidden behind another. Stepped shelves of brick or wood give the same effect.

Ring a sitting-out place with flowers. Set out a few chairs or a mat on the roof and place pots of chrysanthemums around them. The spot where the family drinks tea in the winter afternoon sun becomes a small courtyard enclosed by flowers.

Add light. LED lights can make the roof lovely in the evening. Strings of small, low-power lights along the railing or beside the pots let the flowers be seen after dark. But as noted above, chrysanthemums are sensitive to day length; bright light shining on the plants all night during the budding season can delay flowering. So point the lights at the paths or the seating rather than the plants, and switch them off at a set time.

Chrysanthemums are not only for looking at in pots. They are prized as cut flowers too, because they stay fresh in a vase for a long time. The best time to cut them is morning or late afternoon, when the sun is gentle. Once the outer petals have fully opened, cut the flowers with a long stem using sharp scissors and put them straight into clean water. Changing the vase water every day or two makes them last longer still. That way the colours of winter travel from the roof garden to the table indoors.

Chrysanthemums can also share the roof with other winter flowers. Marigolds, dahlias, roses and petunias go well with them and add variety of colour and shape. A few pots of mint, coriander or lettuce here and there make the garden useful in the kitchen as well as pleasant to look at.

Before starting a rooftop garden, think about the building as well. Many soil-filled pots and the water they hold put weight on a roof, so before planning a large garden it is wise to consult an engineer if needed. Setting pots on bricks or stands rather than directly on the roof surface stops water pooling underneath and protects the roof's finish. Keep an eye on the roof drains so they are not blocked by fallen leaves or soil. And in the rainy season, empty any water standing in saucers, broken containers or anywhere else, because Aedes mosquitoes lay their eggs in exactly such clean, still water.

Decorating a roof with chrysanthemums does more than add beauty; it is good for peace of mind and health too. A little time each morning or evening spent caring for the plants, watering, trimming dry leaves, looking for new buds, offers a pause in busy city life. Children in the family learn to recognise plants and build a bond with nature.

A rooftop garden builds ties between neighbours too. Give a sucker or a cutting from your own plants to someone next door, and a new garden can begin there. In this way the chrysanthemums of one roof can slowly spread across the roofs of a whole neighbourhood, adding a little more colour to a grey city.

Plant a few seedlings in October and the roof can be full of colour by December. When flowering ends, the cut-back plants provide next year's plants, so a small outlay once keeps the garden going year after year. With a little care and a little planning, a grey concrete roof can become a green and colourful refuge.

Photo: Chrysanthemum × morifolium, Shuvaev / Wikimedia Commons (CC BY-SA 4.0)