Advice on Planting Green and for Biodiversity, from Nic Guerra (Barbican Gardener) and the Barbican Wildlife Group

Planting in Barbican troughs

The trough should be lined with BEO black plastic refuse bags – three to a trough – with three1cm holes punched at intervals per sack. The holes will prevent water clogging the compost and using bags help with refreshing the compost, as the bags can be lifted out of the trough separately.
Place small stones, broken terracotta pots, polystyrene packaging, cat litter, gravel etc, to a depth of 5 cm at the bottom of each plastic bag. This helps retain moisture and again prevents water clogging the compost.
Use the best quality affordable multi-purpose peat free compost. Peat comes from peat bogs, which are unsustainable, as well as being homes to wildlife. Expect to need at least 20 litres per black bag and fill each bag up to 2cm from the top of the trough.
Choose plants you like, as long as these benefit bees and butterflies and, of course, the kitchen. Easy maintenance plants that don’t need a lot of watering are perhaps the most practical choice, as even when it rains, balconies don’t get wet. Suggestions for plants are below but try not to buy plants which have their roots growing out of their pots, as this is a sign of an old plant getting pot-bound.
Water the plants whilst still in the pots they came in, at least half an hour before replanting, which makes for easier removal. Pots can be squeezed gently to assist and, once out of the pot, the roots can be very gently massaged apart between finger and thumb. Before replanting, the compost should be watered so it is damp to touch but not soggy.
Lay out the plants how you would like the trough to look, bearing in mind the advised ultimate size. But trailing plants should go along the front of the trough, to grow over the balconies. Scoop out a hole in the compost a little wider than the circumference of the root clump and no deeper than the base of the stem. Put the plant in the hole and then fill in the rest of the hole with compost pushing it gently but firmly around the plant up to the base of its stem.
When satisfied the plant is firmly located, finish off the planting with a gentle sprinkling of water around the plant, avoiding the foliage, if possible.

Plant care

If a good quality compost has been used, good quality plants should not require feeding during the growing season. However, when washing out milk cartons with tap water, the contents can be used for watering plants. A few torn-up tea bags and coffee grounds, dug into the compost, won’t go amiss either.
Any brown looking leaves should be removed, especially before watering or if they’ve fallen on the compost, as this will prevent disease. If you do see black or green fly, aphids etc. and really don’t want them, just pick them off and rub between finger and thumb. A weak solution of washing up liquid – preferably Ecover or equivalent – can be gently sprayed on the leaves but please don’t use any other chemicals.
Watering can take time and it’s best to water regularly rather than drenching. Ideally, watering should be done in the morning or evening, in any event not when the sun is at its hottest. The best method is to water slowly from one end to the other end and then returning. A good test of whether the watering has been sufficient, is to push a finger 2cm into the compost to make sure it’s damp all the way down but don’t water so that it splashes up onto the stem or leaves.
When blooms have faded or died, snip them off, as this may encourage new growth. Some leaves should be left on herbs to enable them to flower and provide food for bees and butterflies.
In an ideal world, the compost should be replaced after every growing season but this would be expensive, so perhaps replace a third each year. However, digging in fresh compost with the old and mixing well would be fine, as long as the compost isn’t compacted. Spent compost must be placed in the black rubbish bags for disposal as general waste.

What you need – some suggestions

Tools and equipment:
A watering can, gardening trowel, spray bottle and gardening gloves – to avoid the feel of compost – are ideal but not essential. A jug, spoon, an empty washing up liquid bottle and resistance to the feel of compost will suffice!

Annuals – plants that generally have a single season life-span:
Cosmos; Petunia; Sunflower; Cornflower; Nasturtium; Calendula; and, of course, Geranium but not pelargonium as these are sterile and of no benefit to biodiversity. Choose varieties such as Geranium Roxanne; Renardii; Pratense; Orion; Mavis Simpson; and Ballerina, all of which are sold as perennials.

Perennials – plants that have a life-span of more than one season:
Aster; Sedum; Pinks; Lady’s Mantle; Ivy; Honeysuckle; Vinca; Wallflower; Lavender; Scabious; Phlox; Helenium; Campanulas: and Salvias.

Herbs – all perennials:
Rosemary; Sage; Thyme; Marjoram; Chives; Mint (best left in pot to control roots); and Tarragon.

If buying online, there are several retailers selling peat free compost as well as nurseries selling plants grown in peat free compost.

The kestrel which has made the Barbican Wildlife Garden her home. Thanks to BenM @StargazerBen for the photo.