So, it’s finally dawning on California that there’s a water shortage. Better late than never, I suppose. But where does all the water go? Well, spectacularly inefficient irrigation of crops and golf courses, not to mention gardens, is one way it gets wasted. Like watering wheat - wheat! - above (click to enlarge)

I remember, years ago, the PR guy from one of Britain’s top garden watering companies telling me – in a tipsy moment after a press party – that 85% of the water that came out of his company’s sprinklers evaporated. Wasted. Gone. Vanished into thin air.

Research at the University of California (Davis) points out that in the southern deserts of the USA 36 inches of water per acre is typically used to grow a lettuce crop, that’s about one million US gallons per acre. Let’s say a field produces two crops of lettuce a year (an estimate probably on the low side), that’s two million gallons of water per acre per year. The water is applied from overhead sprinklers and let’s say that my tipsy PR guy overestimated the wastage, let’s say it’s only 50%. That’s a million gallons per acre per year – wasted. And the most recent figures (2012) show that California harvests about 284,000 acres of lettuce a year.

Across the border in Nevada, Google Earth reveals the patterns created by huge sprinklers - the booms can be up to half a mile long - in a naturally arid region (see below, click to enlarge).

And what about golf courses? According to the United States Golf Association “golf courses in hot, dry climates may require as much as 6 acre-feet of water per acre per year”. As it happens, this curious measure translates into almost two million gallons of water per acre per year for a dry country golf course. If we say a golf course is about 100 acres in total  - well, you get the message.

So, co-incidentally, it takes about the same amount of water to keep a golf course lush and green as it does to grow two crops of lettuce – and most of it is wasted!

There are a number of conclusions to be drawn from all this. And one of them is that California’s new rule that restaurants must ask customers if they would like a glass of water before serving it is not going to solve the problem.

As gardeners, we should abandon sprinklers and install soaker hose watering instead in the shade garden, left, click to enlarge). That’s easy. As for lawns? I’m sure I don’t need to tell you. Lettuce growers, too, should use some form of furrow irrigation, many already do. Or soaker hose – and using soaker hose would also work wonders for the car tire recycling business (that’s what soaker hose is made from). Installation would be a major capital expense, of course, but charges for water use would be cut significantly as growers would use so much less – where they pay for it at all(!) And hey, we’d have to pay more for our lettuce. What’s wrong with that? Lettuce is cheap. And perhaps more crops would be grown in areas with a higher natural rainfall. It makes no sense to waste water growing lettuce in California then truck it to supermarkets in New York. But don’t forget: 80% of California’s water is used by farms and farms are not included in the mandatory 25% cut back.

I'm not sure that reducing 20% of water use by 25% is going to make much difference - that's 5% of the total use. A drop in the... you get the picture: sounds good, doesn't mean much.

And golf courses? That’s a tricky one. The men people who make the decisions about this probably agree it all out on the golf course - and we wouldn’t want to upset them, would we…?

The five volume Trees and Shrubs Hardy in the British Isles by W. J. Bean, usually referred to simply as “Bean”, is a monumental work running to over 4,000 pages. It does what it says: it describes in detail the woody plants (including climbers) that can reasonably be expected to grow outside in Britain (mostly zone 8, some zone 9).

The four A-Z volumes were last revised almost forty years ago, then a supplement appeared in 1988 (see below, click to enlarge), so it does not include recent classification and name changes and recent introductions. Otherwise, it's impressively comprehensive with good descriptions and boundless information on origins and differences between similar plants. It’s invaluable.

Now you can read it - free.

In the first part of a two part initiative, the International Dendrology Society has published the whole thing – all 4,027 pages of it – online. And it’s free: no charge for access. The original four A-Z volumes plus the supplement are currently priced on abebooks.com at £325/$504. Did I mention that the online version is free?

The new online version is easy to navigate and attractively presented. The next step is adding pictures.

You can read more about it on the excellent blog post by John Grimshaw, who’s been heavily involved with the project - and I see he’s had the same idea of including an image of his five volume set as I did!

Take a look at Bean's Trees and Shrubs online - it's invaluable, and it's free.



And what's coming next? Britain's Alpine Garden Society is well into the process of making its invaluable two volume Encyclopaedia Of Alpines available online. It's currently available from abebooks.com for £150/$250.74. You can track the progress of the operation here.

Back in March, I posted a piece here titled New US hardiness zone map launches soon!! Well, it hasn't launched yet, I'm sad to say, but in the current issue of the RHS magazine The Plantsman there are two articles which explore the whole issue of hardiness and hardiness ratings in more detail.

The first is by Scottish plantsman and rhododendron guru Kenneth Cox entitled Evaluating Plant Hardiness. The second is by me and is entitled An International Perspective on Hardiness Ratings. Click each title and the article will appear in a new window

As our climate continues to change, as horticulture becomes more internationally connected, and as gardeners become ever more eager to stretch the limits of what they grow, plant hardiness and how to make clear to gardeners which plants are hardy in which areas becomes an ever more important issue. These two articles make an interesting contribution to the continuing debate.

I arrived back in Pennsylvania the day before yesterday, after a very trying flight from London I have to say, to find judy hard at work in the garden. She was protecting the fresh new growth of so many perennials and shrubs against the forecast frost by spreading old sheets over the plants. It had worked in previous years, and for the previous few days, so she was at it again.

Every old sheet in the house was pressed into service, in fact so many sheets were dragged out that I feared my first night back would be spent in a sleeping bag. Anyway, it worked again. But as the garden expands I think we’re going to have to order a roll of fleece (garden fabric, to US gardeners).

As it happens, just a few days earlier I’d taken a look at the new South Africa Landscape being installed outside the British Museum in Bloomsbury, central London. And with frost forecast there too, they adopted the same approach. In spite of global warming and the advantageous microclimate of central London, Bloomsbury is still not quite like that of South Africa. That’s not to say that a taxi was sent round to the Director’s house to roll him out of bed and collect his bed sheets. The Royal Botanic Gardens Kew (Kew Gardens to most of us) were planting the display and used the same fleece we should be buying for our Pennsylvania garden.

The idea of the planting at the British Museum, by the way, is to highlight the extraordinary diversity of plant life native to South Africa’s Cape region and to make connections between plants, people and the objects on display in the museum’s Africa galleries. Sounds fascinating. The display opens on Thursday.

Find out more about the South Africa Landscape at the British Museum.

One of the first things I do when I get back to England is check out the acacia that’s growing near my daughter’s house in south London. It’s 12-15ft high and is planted in the grounds of the local school – but this is first year I’ve actually been here at just the right time to actually see it flower. It’s quite a sight.

And it’s a sign of how the climate’s has changed. When I was training at Kew Gardens decades ago, the acacias were either growing in a greenhouse or trained against a sunny wall. Now, only about ten miles from Kew, this one is thriving outside and last winter - one of the coldest for a very long time – does not seem to have hurt it at all.

And it really is quite a spectacle.

At last, the promised land is in sight!

The USDA (United States Department of Agriculture) is set to launch their new updated hardiness zone map soon. Yippee! We’ve been waiting for quite a few years now, the last update was in 1990 (left). So it’s great news that the new map, based on the latest climate data from more weather stations than before, will soon be with us. British gardeners take note: the Royal Horticultural Society is also currently looking at the whole issue of rating plant hardiness. So read on to get a sense of the issues.

So: “The Agricultural Research Service (ARS) is in the last stages of getting the map ready for release,’ Kim Kaplan of the USDA told me. “The new map is an interactive GIS-based (Geographic Information Systems) map designed to be web friendly. It will also be presented on the web as jpgs so those without broadband access will still be able to use the map.” They’re just sorting out a service to host the new map so that the vast flood of users doesn’t crash the USDA servers. And you’ll see why this has to be a web based service.

It’s going to be far more than just a printed map, with a number of positive changes. In particular, using a web interface will make it possible to see the boundaries between one zone and the next in far greater detail. The ARS has also developed a way of presenting hardiness zones effectively in areas lacking weather stations, especially in rugged parts of the Western United States.

“As a result,” says Kim Kaplan, “the mitigation of extreme low temperatures by nearby, large bodies of water will be visible for the first time, as will the presence of cold sheltered valleys and mountain tops, which are caused by elevation increases and other geographic features. The warming effect of asphalt and concrete of major urban areas will also be visible in many locations.” So, for the first time, the warming effect of cities will be reflected in the hardiness zones map. Isn’t that great?

And all this is based on the most up-to-date weather station data possible. “The 1990 map was based on weather data from 1974 to 1986,” says Kim Kaplan. “The new one will be based on weather data from 1976 through 2005. Also, more weather stations in the mountains of the western United States will be used in the new map.”

There will be no changes to the zones themselves, except an addition at the warm end. “The 1990 map had zones 1-11 a & b,” says Kim. “The new map will have zones 1-14 a & b…  Zones 12-14… have been added to the legend to allow tropical plant breeders and nurseries to provide zone information on when to bring such plants in, such as from the deck.”

The result is a map of twenty-two zones and subzones in steps of just 5F. That really is the sort of detail that all gardeners appreciate.

Of course, this whole system can never be more than a guide. Other factors like soil drainage, exposure to cold winds are also important and snow cover. But, at last, we’re going to have a zone map that reflects recent changes in our climate. So you may find that you’re now in a warmer zone than you thought – and you can grow far more plants than you thought. Perhaps I can leave my bananas outside all winter in Pennsylvania (on the other hand…)

Have no fear: As soon as the new map goes live I’ll let you know.

This is just so good… I’m so excited… after twenty years we’re going to get a new hardiness zone map... This great… Perhaps I can grow more plants than I thought… etc etc…

(Sorry, I'm a bit lost for images considering the map's not actually out yet...)

I’ve been thinking the muddled issue of hardiness zones this week – and muddled they are indeed. These are ratings that classify plants according to the climatic conditions they’ll tolerate – in particular, how much winter cold they’ll take. Maps divide the world zones according to the prevailing conditions.

American gardeners use them all the time, and for their interest here’s a map of Britain (click to enlarge) split up according to the USDA system widely used in the USA. British gardeners will find the, of course, far more complicated US map below.

The problem is this: At a time when plants (and sometimes their coloured labels) move around the world so quickly, when most plant and garden books are published internationally and with everyone looking up plants online wouldn’t it be more helpful to have one single system?

Britain The RHS has a four zone system (some use an extra zone for parts of Scotland) based not on minimum temperatures but on the growing conditions plants require. The RHS also often cites the actual temperatures plants will tolerate or uses a system of symbols.
Europe Here the USDA system is sometimes used but the European Garden Flora uses its own seven zone system based mainly on minimum winter temperatures.
United States There are three systems. The USDA (United States Department of Agriculture) system of eleven winter hardiness zones and the similar National Arbor Day Foundation system are based on minimum winter temperatures. There’s also the completely different 45 zone Sunset system, used almost entirely in the west, that also takes summer heat and other factors into account.
Canada Agriculture Canada has created an eight zone system based on minimum winter temperatures, summer rainfall, snow cover and other factors.Australia Here they use a modified version of the USDA system.

I could go on…

So I’m wondering – does it matter? Well, you know what I think. I write online and my books are published all over the place – it would be simple to use just one system.

What do you think? One system for everyone or each to their own? Keep in mind that any system is only going to provide a rough guide.

This new book from the Royal Horticultural Society is quite difficult to review. It may seem to deal with just one important subject but the title, New Gardening, is a portmanteau for an extraordinarily diverse range of techniques and ideas.

The entirely reasonable premise is that we cannot continue to garden the way we once did (and how many would like to) with lush, regularly irrigated borders around broad sweeping regularly irrigated and regularly shaved lawns; spraying chemicals, piling on the fertilizer, ignoring the consequences.

So Matthew Wilson, Curator of the Royal Horticultural Society’s garden at Harlow Carr in Yorkshire, has brought together a range of both familiar and new techniques: composting, use of grit and gravel, permeable membranes, water and wildlife, attracting insects, mulching, choosing plants, new ideas on pruning and planting... all with one aim, as the subtitle puts it: “How to garden in a changing climate”.

There’s a huge range of excellent practical pictures, including useful step-by-step series on projects like tree planting the right way and how to make a green roof. There are planting ideas with a drier climate in mind. And while espousing and explaining this new approach he never forgets that the point is to create a garden in which to relax and enjoy the plants and their seasons. And while initially written for British gardeners North American gardeners will also find plenty of useful ideas and advice.

This invaluable book, which recently won the 2007 Practical Book of the Year Award from the British Garden Writers’ Guild, reveals a different approach for a new age, explaining - and showing with some excellent photography - how to re-invent gardening for our new world.

RHS New Gardening by Matthew Wilson is published by Mitchell Beazley.

You can buy New Gardening at a discounted price in Britain here.

You can buy New Gardening at a discounted price in North America here.

I've had a couple of angry emails - rather than public comments - from American readers about my remarks on global warming. Basically saying it's all a fuss about nothing and why am I supporting the unpatriotic lefty pinkos etc etc.

Well, the recent report was the result of a consensus amongst a large group of scientists - and so, necessarily, rather conservative; they all had to agree, after all.

Then the politicians waded in with the scissors. American political representatives cut this crucial sentence: "North America is expected to experience locally severe economic damage, plus substantial ecosystem, social and cultural disruption from climate change related events." It could hardly be more clear.

You can download the full report and you can also read this piece on the political and other influences on the science in the report, and in other coverage, from The Guardian.

Next time... back to plants in the garden, perhaps.