This week, as promised, we will take a look at the different soil depths required for growing various types of plants — edible and ornamental — in beds and borders with special emphasis on those somewhat controversial ‘raised beds’ which have become popular of late.

Raised beds are useful garden additions when constructed on top of otherwise uncultivable land / cemented areas or for plant species requiring specialised growing conditions, such as first rate drainage to prevent species-specific problems. But, as they are prone to dry out quickly, they are not, otherwise, a good idea in hot climates or in locations where water is in short supply.

If, however, raised beds are what you need in order to create a plantable garden then the following guide should help you plan: the same planting depths also apply to the amount of soil / organic compost needed in ‘ground level’ beds and borders for species-specific, optimum plant growth.

They look great but raised beds and borders need careful planning and can be an expensive venture

• A soil depth of six to 12 inches is sufficient for the cultivation of most annual / seasonal flowers except for very tall growing species such as sun flowers, cosmos, Queen Anne’s lace and hollyhocks which require a soil depth of 18-24 inches at least with 30+ inches being more ideal. It is also suitable for ‘small’ bulbous plants like Dutch hyacinths, grape hyacinths, anemones, crocus and dwarf varieties of daffodils and narcissus.

This shallow soil depth, providing that the soil / compost is very fertile, is enough for vegetables such as lettuce, leaf beet / Swiss chard, small, round radish, spinach, green onions, endive, giant red mustard, mizuna and other leafy greens.

Low growing herbs, including thyme, will also thrive.

• A soil depth of 12-18 inches is good for antirrhinums of medium height, dwarf dahlias, godetia and even dwarf roses.

Vegetables and fruits enjoying this depth, include, strawberries, Chinese gooseberries, broccoli, cabbage, Brussels sprouts, cauliflower, kale, onions, leeks, dwarf sweet corn, dwarf peas and beans, dwarf / bush tomatoes and ‘regular’ chillies.

Herbs for this depth include: coriander, chives, garlic chives, borage, oregano, marjoram, mint and basil.

• A soil depth of 18-24 inches is needed for tall growing dahlias, Queen Anne’s lace, hollyhocks, lupins, lavender, sweet peas and bulbs such as amaryllis, lilies, daffodils, tulips and Dutch iris.

Natural stone is best

Climbing beans and peas, small and medium sized carrots, turnips, khol rabi, celeraic, celery, courgettes / zucchini, cantaloupe melons, medium height tomatoes, capsicums, aubergines, tindas, sweet corn, beetroot and long radish / mooli.

Medium to tall growing herbs with the exception of rosemary and sage which both tend to be very deep rooted. • A soil depth of 24-36 inches: very tall growing flowers of both annual / seasonal and perennial types including bush roses.

Potatoes, parsnips, tall varieties of tomatoes, water melons, pumpkins and winter squash, lauki and other related vines, asparagus, artichokes, main crop, large carrots and sweet potatoes.

Herbs: sage, tall agastache, achillea, orach, lemon grass and tarragon.

Climbing roses, other flowering climbers and medium tall shrubs need a planting depth of 36+ inches, as do fruiting vines such as grapes, passion fruit and kiwi fruit.

Dwarf species of fruit trees do best in 36-48 inches of good soil / organic compost.

Chico and sharifa like 48+ inches.

Other fruit trees, oranges and lemons for example, require at least 48 inches of fertile top soil if they are to thrive and fruit and the same applies to apricots, peaches, plums, pears, etc.

Trees in general, unless being cultivated as relatively ‘small’ ornamentals, are not particularly suitable for growing in restricted beds / borders of any kind as, aside from being extremely deep rooted, they can seriously inhibit the well being of other plants being cultivated in the immediate vicinity.

Points to ponder: Constructing raised beds, quite irrespective of their depth, can be a costly business all depending on the type / quality of construction material used, plus, the amount of sweet earth / organic compost to be bought in to fill them with. There are also drainage pipes to be taken into account for ‘high-rise’ raised beds being built from scratch on top of existing, solid surfaces such as rock or cement. Run-off water has to go somewhere so it may also be necessary to connect drainage pipes to other, possibly existing, drains and these will require easily accessible filters, to be cleaned on a regular basis, to prevent soil / debris from blocking them up.

In locations where termites are problematic, the use of wood in raised bed construction or as border edging is not recommended: termites feast on wood and somehow, they ‘smell’ its presence from surprisingly far away.

Plastic, no matter how strong and durable it is claimed to be, should also be avoided. It soon becomes unsightly, is both hot / cold depending on the season and will need replacing every two to three years or even less if the quality is poor.

Bricks / blocks and mortar can be expensive and natural stone, the best building / border edging material of all, is exorbitantly expensive.

If building raised beds, edging ground level beds / borders, please give much time to thinking and planning before making a start. Getting it right first time around saves a fortune, plus, results in happy plants and equally happy gardeners.

Please continue sending your gardening queries to zahrahnasir@hotmail.com. Remember to include your location.

The writer will not respond directly by e-mail. E-mails with attachments will not be opened.

Published in Dawn, Sunday Magazine, July 19th, 2015

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