Rooftop garden tips: cocopeat, mustard cake, neem and hand pollination

On a rooftop in Dhaka in late 2020, a doctor at Dhaka Medical College was growing lemon, pomegranate, guava and mango in tubs and drums, along with long bean, eggplant, gourd, water spinach, Malabar spinach, ridge gourd and papaya, and medicinal plants such as neem and aloe vera. She told The Business Standard that friends' examples had drawn her to rooftop gardening during the coronavirus period, and that her family had bought few vegetables from the market since May.

Not every rooftop does so well. Many gardeners water and tend their tubs faithfully and still get little fruit, often because of small, avoidable mistakes. In December 2024 The Agro News published nine tips for rooftop gardeners. This guide keeps those tips, checks them against research from Sher-e-Bangla Agricultural University (SAU), the Bangladesh Agricultural Research Institute (BARI) and university extension services, and explains how to mix the soil, feed plants in containers, deal with soil fungi and insects, and help flowers turn into fruit. Where a dose matters, it says which authority gives one, and where none does, it sends you to your agriculture office. The same nine tips had appeared in The Daily Ittefaq in December 2021.

Rooftop gardening grew quickly in Bangladesh's capital in the years before this guide. Citing data from the Agriculture Information Service (AIS), The Business Standard reported in November 2020 that Dhaka had about 4.5 lakh roofs covering more than 4,500 hectares, and around 30,000 rooftop gardens, about 7 percent of the roofs.

The Department of Agricultural Extension (DAE) took up a three-year urban agriculture project from 2018 to June 2021 to set up 600 rooftop gardens in Dhaka. By November 2020 its project director said 500 had been established, and that about 6,975 rooftop gardens in Dhaka were under the department's supervision.

Earlier, from December 2015 to December 2017, the Food and Agriculture Organization of the United Nations (FAO) and DAE ran a USD 299,000 project to enhance urban horticulture in Dhaka and Chittagong. According to FAO, it set up 250 demonstration rooftop gardens, trained 800 city dwellers in rooftop garden technology and 45 local youths as gardeners, and prepared a guide on assessing a building's weight capacity, using space, choosing plants and varieties, getting materials and seeds, preparing soil and following a crop calendar.

Universities have trained gardeners too. In 2018 SAU's Department of Agricultural Botany ran a rooftop farming course for more than 200 city dwellers in seven batches, covering soil composition, pesticides, seed sourcing and plant breeding; its model garden for 1,000 to 1,200 square feet cost Tk 40,000. The Business Standard reported in August 2022 that the course had stopped after 2018 for lack of funds, and that there were about five lakh rooftop gardens across Bangladesh.

A study in the Dhaka University Journal of Earth and Environmental Sciences, published on 9 December 2024, estimated that Dhaka had 257.95 hectares of rooftop gardens, 0.32 percent of the city's 80,220 hectares of continuous roofs. It found that these gardens met about 0.3 percent of the city's demand for fresh vegetables and fruit, that gardening households ate about 75 percent of what they grew, and that the gardens contributed about 6 percent of household vegetables and 18 percent of household fruit for the families that kept them.

Policy has followed. In January 2021 Dhaka North City Corporation announced a 10 percent holding tax rebate for houses with rooftop gardens, and in June 2023 the Local Government Ministry announced it would extend the discount to all city corporations and municipal areas, The Financial Express reported.

The first point in the tips is simple: a plant in a tub or drum must be fed. A tree in a field sends roots far through the ground to find water and nutrients; a plant in a container can use only what is in the pot. So the gardener has to supply food, watch for disease and act early, and be patient.

Organic manures release their nutrients slowly. Tamil Nadu Agricultural University's agricultural technology portal gives well-decomposed farmyard manure as containing about 0.5 percent nitrogen, 0.2 percent phosphate (P2O5) and 0.5 percent potash (K2O), and says nutrients in oil cakes become available to crops 7 to 10 days after application, once they are mineralised. That is why feeding in containers works best as a steady routine rather than a single heavy dose.

The tips advise always mixing some cocopeat with the soil. Cocopeat, also called coir pith or coir dust, is the powdery material left from coconut husks. It holds a large amount of water, like a sponge, and is light, so it adds little weight to a roof. With it in the mix, the soil stays moist without turning soggy, and water does not stand at the base of the plant after heavy rain. A soggy base invites many diseases, so the soil should be moist but never waterlogged. The tips also note that seedlings sprout and grow well in cocopeat.

Coir needs some care. A study by C. de Kreij and G.J.L. van Leeuwen in Communications in Soil Science and Plant Analysis (2001) found that coir dust introduced in Dutch horticulture had restricted the growth of some pot plants, because it was high in potassium and sodium and held back calcium and magnesium. After the coir was washed and treated with a solution containing calcium and magnesium, begonia, chrysanthemum and schefflera grew as well in it as in peat. Buy cocopeat that has been washed, or rinse it well before use.

On organic matter, the tips rank leaf compost first, vermicompost second and cow dung third. Leaf compost is scarce and costly, while vermicompost is easy to buy. The original tip suggested at least 40 percent organic fertiliser in the soil mix.

Research on Dhaka rooftops points to smaller shares working well. At SAU's rooftop garden, an experiment from October 2017 to March 2018 compared growing media for BARI Tomato-14. The highest yield, 2.15 kg of fruit per plant, came from a plastic pot filled with 80 percent soil, 10 percent cow dung and 10 percent vermicompost by weight, together with inorganic fertiliser. A second SAU rooftop trial, from May to October 2018, tested BARI Begun-8 brinjal: the highest yield, 740.16 g of fruit per plant, came from a plastic pot with 70 percent soil, 20 percent cow dung and 10 percent vermicompost by weight, along with the recommended dose of inorganic fertiliser. Mixes with 30 and 40 percent cow dung were among those tested.

Whatever the mix, every tub, drum and crate must drain. Holes in the base and a loose, crumbly soil keep roots from sitting in water.

The tips call rotted mustard-cake water a 'biryani' for plants. Mustard cake, khoil in Bengali, is what remains after oil is pressed from mustard seed. Rot it in a little water in a clay pot for at least five days, and seven to fifteen days is better; then add more water before use. It smells, so a little molasses may be added, and a pot kept on the roof keeps the smell out of the house. The tips say not to give it during rain.

Mustard cake is rich in nutrients. A 2017 paper in the Journal of the Sylhet Agricultural University, citing the Bangladesh Agricultural Research Council (1997), gives its nitrogen, phosphorus and potassium content as 5.1 to 5.2, 1.8 to 1.9 and 1.1 to 1.3 percent, and says it also supplies secondary and micronutrients such as sulphur, zinc and boron. The same paper notes that mustard cake makes up about 60 percent of the seed after oil extraction, that oil cakes mineralise slowly but lose much less nitrogen as gas than nitrate fertilisers do, and that oil cakes have been found to reduce nematode development.

The research and extension sources consulted give no dilution rate for liquid mustard-cake feed in pots, so this guide gives none. Ask your metropolitan or upazila agriculture officer how strong a solution to use for your crop, and start mild. The tips also praise pond-bottom mud as a soil addition for plants.

Soil holds many harmful fungi, enough to kill seedlings. The tips advise adding a beneficial-fungus product to the soil while preparing it. Such products are based on Trichoderma, a fungus used to control disease-causing fungi in the soil, and the tips note that it also works like an organic fertiliser and leaves the soil loose and light.

BARI has tested this. In net-house trials at Gazipur from 2010 to 2014, reported in the Bangladesh Journal of Agricultural Research in March 2017, BARI scientists formulated Trichoderma harzianum bio-fungicides on rice bran, wheat bran, grass pea bran and mustard oilcake. All five tested combinations controlled the seedling disease of brinjal caused by Rhizoctonia solani in tray soil and seedbeds, and the treated seedlings grew longer, heavier shoots and roots. Follow the product label and your agriculture officer on how much to mix.

Whatever you grow, start with seed of a good variety. Good seed gives a good crop; poor seed wastes all the care that follows. The tips recommend raising your own seedlings from seed, because there is no guarantee that a seedling bought in the market is of the variety it is said to be.

Seed can be treated against fungus by soaking it for a while in water mixed with a fungicide. Which fungicide and how much depends on the crop, so ask the agriculture office and follow the label.

More plants do not mean more yield. Crowded plants compete for the little food a pot holds. The tips give clear limits:

• One plant in one tub.
• Only two plants in a fruit crate.
• Drainage holes in every crate and tub.
• A slightly deeper container for climbing vegetables.
• For a trellis on a roof, where there are no posts in the ground, tie sticks around the fruit crate.

Harmful insects dislike neem, and the tips advise spraying a neem pesticide on the leaves of all plants once every seven days. The National Pesticide Information Center at Oregon State University describes neem oil as a biopesticide from the seeds of the neem tree. Its main active ingredient, azadirachtin, reduces insect feeding, acts as a repellent and interferes with insect hormones, making it harder for insects to grow and lay eggs.

Neem oil is practically non-toxic to birds, mammals, bees and plants, the centre says, but slightly toxic to fish and other aquatic life and slightly irritating to the eyes and skin. Keep spray away from fish tanks and ponds, and follow the label instructions on dilution and protective clothing.

The tips also suggested a monthly Epsom salt spray. Epsom salt is magnesium sulphate. Linda Chalker-Scott, an extension urban horticulturist at Washington State University, advises home gardeners not to use it, because its benefits apply only to intensive crop production where magnesium is known to be deficient. The University of Washington's Miller Library, summarising her work, recommends compost instead. Use it only if a soil or leaf test shows a lack of magnesium.

For a quick start, the tips suggest stem amaranth (data), Malabar spinach (pui shak), red amaranth (lal shak) and coriander, which can be eaten in about 25 days. Quick harvests encourage new gardeners. Red amaranth should be netted.

If Malabar spinach leaves get spots, the tips advise cutting the spotted leaves, spraying a fungicide on advice, or pulling up the plant and sowing again. They say urea helps Malabar spinach grow fast and DAP helps cucumber. Use fertiliser with care: Alabama Cooperative Extension warns that too much nitrogen causes excess leafy growth that can lead to blossom drop, and that too much of any fertiliser raises salt levels around plants. Cucumber also needs fungicide sprays from time to time.

In strong sun, mulch the base of the plant with special polythene, dry leaves or straw. University of Florida extension notes that mulch keeps soil and roots cooler in summer and slows evaporation, so plants need less water; it advises a layer of 2 to 3 inches of organic mulch.

For the flowering stage, the tips name, by brand, two products to spray on the leaves in the late afternoon after flowering and two groups of fungicide to spray every 15 days. They also advise keeping a micronutrient product at home and spraying it at least once a month. Use only registered products, at the rate on the label and on the agriculture officer's advice.

Flowers can drop because of great heat, rain, lack of food, disease or sudden swings in the weather, and because they were not pollinated. Heat matters most for tomato. Alabama Cooperative Extension, in a peer-reviewed sheet revised in June 2023, says tomato flowers often abort when day temperatures exceed 85 degrees F and nights exceed 72 degrees F, and that temperatures below 55 degrees F can also stop fruit set. Short spells of a week or less should not cause much of a problem. It also advises keeping plants healthy and well watered and giving them at least six hours of full sun every day.

For gourds and cucumbers, poor pollination is a common cause. Washington State University's HortSense explains that unpollinated fruits may start to grow but stay small, rot at the blossom end, turn yellow and drop; hand pollination works well in small gardens. The tips advise rubbing the pollen-bearing part of the male flower onto the female flower. A University of California master gardener's guide adds these details:

• Tell the flowers apart: the male flower has a thin straight stem; the female has a thicker stem with a swollen part, the future fruit, just below the flower.
• Pollinate in the morning, soon after the female flower opens, as it blooms for only a day.
• Brush the male flower's yellow pollen on to the centre of the female flower with a soft brush, or pick the male flower, pull back its petals and touch it to the female flower.
• Repeat for every female flower; the same method works for melons, cucumbers and pumpkins.

When a crop keeps failing, take a leaf or a photograph to an expert. The FAO and DAE project worked through metropolitan agriculture officers and sub-assistant agriculture officers in Dhaka and Chittagong, and the upazila or metropolitan agriculture office is the place to ask about pests, fertilisers and pesticide doses.

Photo: AbuSayeed / Wikimedia Commons (CC BY-SA 4.0)

Sources: The Agro News; The Daily Ittefaq (18 December 2021); The Business Standard (16 November 2020; 1 August 2022); FAO Bangladesh, project TCP/BGD/3503; Dhaka University Journal of Earth and Environmental Sciences (9 December 2024); The Financial Express (14 June 2023); Sher-e-Bangla Agricultural University rooftop trials (2017-2018); Journal of the Sylhet Agricultural University (2017); Bangladesh Journal of Agricultural Research (March 2017); Communications in Soil Science and Plant Analysis (2001); Tamil Nadu Agricultural University Agritech Portal; National Pesticide Information Center (2012); Washington State University Extension; Alabama Cooperative Extension System (2023); University of California Cooperative Extension (2019); University of Florida IFAS Extension.