Roof Gardening

How we turned out back garden from this...

... to this...

The basics

Roof gardening has challenges over and above gardening on the ground. Not only does everything have to be grown in containers, but everything needs to stand on the roof of a building – and don’t forget that the roof of the building may be the floor of your garden but it’s also the ceiling of the people below.

We are lucky enough to have some outdoor space with our flat in London, and we’ve spent the past few years turning a couple of empty, flat terraces into lush green gardens.

Our front garden is a more traditional roof garden – Mediterranean trees in pots against the wall and a screen of bamboo to shelter delicate plants from the worst of the wind. This garden was already planted to some degree when we moved in, and our work has been to add to what was already there.

The back garden was, as you can see from the photo above, literally an empty concrete terrace when we moved in, and so we attempted something more ambitious. We have tried to create a traditional English country garden on the roof of a building in London.

Speading the load

Shrubs are heavier than you’d think; trees heavier again. Soil is heavy; wet soil even heavier. So avoid the temptation to add to this with heavy containers.

Our back garden has around fifty containers in it. If each one were made of terracotta or stone, then we’d be adding considerably to the weight. We have used light-weight plastic containers and faced the front of the flower bed with a decorative wooden construction. All of the tubs are the same make and size meaning that by swapping two tubs we can move a plant without disturbance to the roots and without changing the overall look of the garden.

There are other ways to save weight too. We have replaced all of the bark – which can become soggy and heavy when wet – with rubber “fake bark”. You don’t notice the difference except that it doesn’t hold water and you can walk on it in bare feet without pain.

Generally, the strongest part of the roof will be at the edges near the building walls, but in any case all roofs are not made equal. We’re lucky that the roof on which we’re constructing is strong but if there’s any doubt in your mind at all – and there probably should be – about the weight bearing potential of the roof, then best to get a structural surveyor in to advise you.

Once the garden is planted, don’t forget that your plants and trees will grow. What started as lovely little seedling can quite quickly turn into a full-sized tree. Not only will that tree be much heavier than you planned for originally, but it’s also going to require much more water, which brings us nicely onto...

Water water everywhere

Roof gardens need water. As container gardens, you can’t rely on the rain to keep them moist and healthy. A container will be drained of water by a growing plant more quickly than you may realise. A plant in the ground can send roots further down beyond the dry summer soil, but a container plant does not have that option. There are specialist container composts on the market, but they are no substitute for regular watering.

We have automatic watering systems in the garden. It’s expensive to buy initially, and takes some time to run individual pipes to each plant, but it means that even when we’re away we know that each plant is getting enough water and getting it regularly.

Automatic watering systems also avoid soaking the roof with water which the plants don’t need. If the balance is absolutely right, then the watering system should keep the soil in the container wet enough for the plant to thrive but with only a minimal amount of water ever pouring through the container – it’s not only flooding the roof, it’s a waste of water.

Roofs are designed to be rain-proof; they are not designed to have pools of water sitting on top of them for extended periods. Don’t presume that because the roof is rain proof, you can afford to drench it daily.

When rain falls, it will water the plants, but you should let it fall onto the roof too. Avoid the temptation to cover the roof in plastic to waterproof it. The roof is designed to take rainfall and safely get that rainfall into gutters and away. Make sure that there’s a suitable air gap between the surface of the terrace and the containers – a few inches for larger containers – so that any water which does pool on the surface can evaporate naturally. If you can, try to make your garden invisible to the rain falling on the roof. The rain should fall onto the roof just as it would if your garden weren’t there.

It is possible to collect rainwater in a water butt and recycle that for watering the garden. But don’t rely on that alone. If there are several days without rain then not only will the plants have been left dry and need the water, but the water butt will not be refilled. It will be very hard work to have a verdant, lush roof garden without using some mains water to keep it so – if this offends your green principles then best to go for draught tolerant plants only and have fewer plants than let your leafy garden die a slow death. You don’t have to be a eco-destroyer to have a roof garden, though. A decent watering system will minimise the water you use and a water butt can be connected to a watering system.

Saving space

Roof gardens are small. There won’t be space to have a sea of daffodils nodding in the spring or a woodland full of bluebells in summer and cyclamen in winter. But there are a few things you can do to to save space.

Our front garden is covered in decking. Not only does this provide the air gap above the terrace floor to allow for evaporation, but also provides useful storage space. Bear in mind that the air gap needs to remain, but even so, you can sneak a few things underneath for storage. We store all our spare plant pots and lengths of wood for repairs underneath a decking area in our back garden, for instance.

But it’s not just spare plant pots that can go under decking. We wanted to cover a fence with some climbing plants, but to put some containers at the foot of the fence would have ruined the ability to stand at the edge of the garden looking at the view, so we put the containers under the decking. Using a hole saw we made a circular hole in the decking board and carefully fed the plant through so that the roots were below the decking. Then we potted the roots up and left the plant growing above the decking.

But even if you can’t create more space, you can create the illusion of more space. Plant in layers so that wherever you stand in the garden, you’re looking beyond one plant to another. But remember to plant the winter-flowering plants where you can see them from the window – don’t kid yourself that you’ll wrap up warm to go out and admire a winter-flowering jasmine in the January snows. You won’t, and for the rest of the year it will be green and dull. Plant it up near a window and you won’t have to brave the cold to see the yellow flowers bobbing in the frost.

A couple of words of warning on this technique, though. Firstly, some plants don’t like it. Russian Vines thrive in this arrangement, Clematis seems happy, but Honeysuckle did not enjoy it at all. Secondly, remember that if the container is underneath the decking, it won’t get much water even if it has been raining. You’ll need to ensure that these containers are watered even if you have to go out in the rain to do it.

Trees and flowers

Once all the engineering is done, it’s time to do the fun part – buying and planting some plants.

Roof gardens tend to be windy. You’ll be able to find a few sheltered corners to grow delicate things, but generally you’ll want plants which can take a drying wind without withering. Wisteria will die instantly a cold dry wind hits it, but grasses will generally stand up to the worst gales. You should avoid plants which have thin, dry leaves. Plants with thick, water-filled leaves are good. Olive trees will take the winds, for instance.

Think about size too. Trees are fine on a roof garden, but plant species which can be heavily pruned if necessary. We have Twisted Willow trees growing. Not only because they are tolerant of wind, but because they grow into tree-like shapes quite quickly and if they become too large, will happily be hacked to pieces and thrive on it. Word of warning on willows, though – water water water. That’s three words. A willow will grow brilliantly with enough water, but will wither within days if left to dry. A partially dried out willow can be revived with a heavy soaking, but best not to let it get to that position in the first place.

We have other trees, too. We have a hawthorn in a sheltered spot and a olive tree against a south-facing wall. We did try a Silver Birch which grew well for the first year or two, but a fault in the watering system left it without for a little too long and it never recovered. The willows came back bushy and green as soon as the water hit them again. Bamboo can suffer too. Our black bamboo took two years to fully recover from a period of draught. For some plants, too much water is never a problem.

The fact that our garden is in containers means that plants can be grown together regardless of soil type. We have a Camelia in a sheltered spot growing in amongst the Roses and Hydrangea which would be hard to achieve in a traditional garden.

Some plants insist on their tap root going straight down; these plants will never thrive in a container. But most plants – including Roses and most traditional cottage garden shrubs – will grow well as long as sufficient nutrients and water are given. Don’t underestimate the size of a plant you’ll be able to grow in a relatively small container.

Roots can escape from containers though – keep a close eye for any roots which may sneak through the bottom of a container and start making their way through the darkness under the containers. If you do find any, then simply cut them off – if the plant suffers as a result then it was the wrong plant for that location – but mostly the plant will not suffer at all.

I often post on Twitter about growing edibles in the garden. The truth is that we’ll never grow enough to be self-sufficient on a roof garden, but it is fun to be able to wander around the garden and pick a fresh raspberry from a cane or fig from a bush. Having said that, we did grow a huge quantity of runner beans this year – so it’s something we may experiment with in future.

We even have a lemon tree growing. Along with a few other more delicate plants, it gets moved into a nice protected spot over the winter and given a protective coat of fleece to see it through the worst, but during the summer months it grows well and produces a couple of lemons a year. There’s no point in having delicate plants which put on their best show in the winter months though. We have some Nandina Domestica which put on their show of just as the weather is turning cold. After two years of wrapping them in fleece for the winter, we finally did the sensible thing and moved them to somewhere warmer and sheltered so we can actually get to see the flowers and berries when they happen.

We have some miniature fruit trees, too. The diminutive size of the branches means that you’ll need to prune the crop quite heavily to avoid the tree being overloaded, but we do get enough apples and pears for one crumble a year.

Selecting plants for a roof garden is trial and error. Despite all the books on the subject, you’ll find some things which on paper should thrive will be dead within a year and sometimes something supposedly delicate will flourish despite the full force of a northern breeze. We’ve had a few failures over the years. Our original Wisteria died in the first cold gale; a replacement in a more sheltered spot has turned into a monster though. Our blueberry bushes died after two years, and the kiwi fruit has never matched its impressive first year growth again.

But amongst the failures, there have been many successes. Agapanthus love being in a container and within a few years, we have flower talks five foot high swaying in the summer breeze. Bulbs are a good option too. They can be easily squeezed in the corner of a container and a sack of mixed daffodil bulbs spread around the place can give colour to an otherwise dull time of year in the garden.

Some plants have both thrived and failed. We planted two Campanula. One has self-seeded and for two weeks a year provides an electric blue carpet over large parts of the garden. The other died within a month without so much as a single flower.

So if you have a flat roof which can take the weight and water of some containers, I’d say “go for it” – it’s amazing what you can achieve with a few bags of compost and a trip to the local garden centre.