Q. We are creating a food forest in our Lahore garden but have a problem with water. We have a borehole but want to recycle water from our septic tank to irrigate the food forest, which currently contains mangoes, lemons, oranges, pomegranates, amrud, kachnar, etc. The problem is that we do not want the recycled water to seep into the groundwater and the borehole and contaminate them. Is it possible to purify the water from the septic tank before recycling it for irrigation? I understand that some plants can act as water purifiers. Please suggest some common plants for this purpose and explain how they should be utilised.

A. The water from your septic tank is known as ‘black water’ which, without specialised treatment, combined with regular testing for dangerous bacteria/pathogens, should not be used on food crops of any kind. Water purification processes of septic tank liquids, by means of ponds and specific plants do exist, but such systems require a level of expertise that the writer does not, unfortunately, possess. It is not advisable to simply search for supposedly ‘safe’ examples on the internet and replicate them. What is needed is reputable, qualified guidance, and I suggest that you contact the Arid Agriculture Research University, Rawalpindi for this.

Q. We have roof space available for a garden and wish to have a vegetable patch, some climbing vines and flower beds, etc. The maximum soil depth will be about 15 inches. Is this deep enough for the vegetables and plants we want to grow? The entire growing area will be coated with two layers of coal tar/bitumen before making growing beds and adding soil. Will this be sufficient to protect the concrete roof below? We reside in Karachi.

A. A soil depth of 15 inches is fine for many vegetables but not for potatoes and other relatively deep rooted crops. Most perennial vines and perennial shrubs would also require more soil depth than this but annual climbers — such as sweet peas, peas, beans, climbing nasturtiums and climbing members of the squash family — should be fine. The waterproofing you mention may be okay, but it could possibly be improved by the addition of waterproof membrane laid over the coal tar/bitumen as the latter melts and becomes sticky in summer heat. Please discuss this with a roofing expert. Provision of suitable drainage, capable of withstanding monsoon rains, is an additional necessity that should be incorporated into your plan.

Q. A lot of sowing guides online instruct putting an airtight film on the pot to expedite seed germination. What’s the science behind it and would you recommend doing so? Should I temporarily remove the film at intervals?

A. The airtight film is for retention of soil warmth and humidity and, in our often overly humid climate, simply encourages seedlings to ‘damp off’ or rot and die. Online seed sowing guides are usually aimed at gardeners living in very different climates than ours and so are not, on the whole, applicable to Pakistan although, as always, there will be exceptions.

All your gardening queries answered here

Q. I want to grow tomatoes, in pots, on my rooftop. Can I do this now and how to proceed?

A. This column of September 16, 2018 should supply all the information you need. Here is the link: https://www.dawn.com/news/1433062/gardening-grow-your-own-tomatoes

Q. Are kiwi fruit, kaffir limes, fittonia and calathea suitable for growing in Karachi? I already have the seeds. When should these be sown?

A. Kiwi fruit needs a winter chill period of approximately six to eight weeks before they will set fruit and are not, therefore, suitable for Karachi. Kaffir limes are worth trying from seed sown either right now or in spring. Fittonia — nerve plant — is eminently suitable for Karachi and you can sow the seed anytime. Calathea is a tropical species that prefers indirect light and warm conditions, so is best kept indoors during the winter months. Seed is best sown in spring.

Q. Are blueberries being grown in Pakistan and, if so, where can I buy plants?

A. Blueberries are being grown in some parts of the country, mainly around Rawalpindi, Islamabad and further north. Plants are occasionally available around Lahore and Islamabad, during late winter to early spring.

Q. Two years ago, I planted two kumquat trees in my garden, near the sea, in Karachi. They are now about four feet tall and are planted in full sun. They have been flowering and fruiting since the beginning but the fruit stays green and doesn’t turn orange. I fertilise and water them but still the fruit does not ripen. Advise please.

A. Kumquat — Citrus japonica fruit — can take months to ripen and, in Karachi, they usually ripen in very early spring. They need at least six hours of sunshine to do well, and it sounds like your trees are performing beautifully and are in good health. All I can suggest is that you wait and see if the fruit finally decides to turn orange in spring.

Please continue sending your gardening queries to zahrahnasir@hotmail.com. Remember to include your location. The writer does not respond directly by email. Emails with attachments will not be opened

Published in Dawn, EOS, September 13th, 2020