If there's one thing that I love about the internet, it's the opportunities it gives to discover the answers to questions that one never got around to asking. Courtesy of a recent unscheduled stopover at the website of the Financial Times , I now know the precise definition of a megacity (a city with a population of over 10 million), as well as the slightly startling prediction that by the year 2050, 75 per cent of the world's population will be urban, many (presumably) living in purpose-built flats and apartments.

Here in Ireland, apartments already account for almost 11 per cent of all household types nationally, while in Dublin the figure is 32.4 per cent. A consequence is that many of us don't have a garden in the conventional sense, but instead, along with allotments and community gardens, increasingly grow our flowers and food in the sky, successfully squeezing pots, tubs, troughs and boxes on to the tiniest of city balconies, roof gardens, sunny window ledges or wherever else we can find the space.

While this sort of container gardening can be hugely rewarding as well as highly productive, it often presents challenges. The good news is that, given a little forward planning, none of these is insurmountable as long as you keep a few important things in mind.

1. Before undertaking any roof garden projects, always check with an architect or structural engineer to find out your roof’s load-bearing capacity. Planning permission may also be required. In the case of balconies, you can get advice on its load-bearing capacity from the original engineer, whose name can be established from the planning file in the local authority planning office or possibly through the apartment management company.

2. Choose containers carefully – the lighter the better. Avoid concrete (too heavy) or terracotta (too porous) and instead go for plastic, metal or fibreglass, always ensuring there are drainage holes at the base.

Size matters: larger containers hold more nutrients, offer greater protection against extremes of temperature and are less likely to dry out. For edible gardens in particular, remember that shallower containers (15-20cm) are fine for salad leaves and many herbs but a deeper container (minimum 45-60cm) allows you to grow a greater range of food crops.

Regular watering is also vital, which is why containers that come with their very own built-in reservoir are a great idea. Suppliers include Sligo-based Quickrop (quickcrop.ie).

3. To grow great plants, you need a great growing medium. Ordinary garden soil on its own isn’t suitable. Nor is a peat-based compost, not only because of environmental considerations but also because once dry, it’s difficult to re-wet and so light that the container becomes top-heavy and prone to blowing over. Instead, use a loam-based, nutrient-rich compost such as Pro-Grow compost from Enrich (see enrich.ie for countrywide stockists), which is a mixture of screened soil, sand and compost rich in organic nutrients that are slowly released over a period of three to five years. Or go for the Rolls Royce option-Carbon Gold All Purpose Biochar compost (available from Quickcrop), which contains coir, biochar (a fine-grained charcoal that helps retain nutrients and encourages root development), mycorrhizal fungi, vermicompost and seaweed .

4. Wind exposure is often a problem. Too strong and it damages soft growth or knocks over containers; too warm and it dries out composts and burns flowers/foliage; too cold and it chills/kills plants outright. Offer protection in the form of semi-permeable screens such as trellises, cloches, fleece, Bionet or containers that come with removable lids, but always ensure they’re fully secured. Growing a few wind-tolerant evergreens amongst other plants also helps; many silver leaved plants (lavender, rosemary, santolina, perovskia) cope well in these conditions as long as they’re in full sun.

5. Maximise space by thinking vertically, using tiered plant containers and window boxes fixed firmly to walls. For containers that are both edible and ornamental, mix herbs, edible flowers (borage, chives, nasturtiums, marigolds), alpine strawberries, salad leaves or compact vegetables, such as cherry tomatoes or dwarf French beans. In protected spots, try growing climbing plants such as ivies, sweet pea, French bean ‘Cobra’, runner beans or even the climbing courgette, ‘Tromboncino’.

6. Balconies can often be in shade for part of the day, in which case low, quick-growing, space-efficient leafy crops such as spinach, lettuce, rocket, oriental salad leaves, parsley, mint and chives are a good choice for edible gardens. Ornamental plants that are both shade and wind-tolerant include

Pinus mugo, Viburnum tinus , Stipa tenuissima

and

Anemanthele lessoniana

.

7. Top up fertility levels when required using a liquid seaweed feed and consider keeping a wormery (see wormery.co.uk or wigglywrigglers.co.uk).

Check out the GIY Get Ireland Growing Fund, a community  initiative developed by GIY in partnership with AIB. The fund is open to applications from community groups, schools and not-for-profit initiatives seeking to develop food-growing projects.  Grants of between €500 and €5,000 are available. Closing date is end of April. See giyireland.com