If there's one thing you can say about a vegetable garden, it's that it's green (in the colourful sense of the word). Salad leaves, root crops, beans and brassicas offer lots of different textures and shapes but, if you're looking for a rainbow, you're looking in the wrong place.
A quick glance through the plant catalogues, however, soon reveals how many colourful vegetables are available and, apart from the visual impact, there are good reasons why they deserve a place in your veg garden.
What makes vegetables red and purple?
First, though, what makes vegetables red or purple (which in plant terms is a deep, deep red)? Well, it's anthocyanins, those anti-oxidants recognised for their health-giving benefits, as mentioned in the post on Pruning Blueberries.
Anthocyanin content affects a plant's energy consumption and water content, and has been shown to reduce levels of photosynthesis. These differences account for some of the advantages listed below. Unfortunately, you can't automatically assume that all purple-coloured varieties possess the same advantages. Characteristics vary, depending on the cultivar and growing conditions. However, if you have to combat drought, slugs or heat, dark-coloured varieties offer likely resistance, as well as other practical advantages.
How often have you combed your cabbages for caterpillars, only to return next day and realise, from the even more ragged condition of the leaves, that you missed some? The green caterpillars of cabbage white butterflies are much easier to spot on purple varieties of brassica such as Red Drumhead cabbage.
In the same vein, purple-podded peas, French beans and mange-tout are much easier to see amongst the green leaves. This in itself helps to prolong the harvest, as pods inadvertently left on the vine will continue to ripen and discourage the production of new pods.
Cold and Drought Resistance
Here's where those anthocyanins come in, lowering the freezing point of plant tissue. Black Jack is a very dark lettuce that copes well with cold, and the purple-tinged January King cabbage is notably hardy.
In the case of drought-resistance, high levels of anthocyanins are associated with a reduction in the rate at which a plant loses water through its leaves. The pepper Pretty Purple, for example, has proved more drought-resistant than its green relatives.
Dark-leaved varieties also tend to grow more slowly than their green cousins. Cultivate several varieties of green and red lettuces, for example, and you'll find your harvest more spread out, making it less likely that you'll face a glut.
Certain types of dark-leaved lettuces are also slower to bolt (when a plant stops leaf production and goes all out to produce flowers and seeds) which is helpful if you face summer temperatures above 85F (29C).
Although The Lettuce Bolting Resistance Project, run by Colorado State University, found Batavian lettuces to be most bolt-resistant, whatever their colour, the red leaf lettuces that they tested showed less likelihood of bolting, compared with green leaf lettuces.
A purple tinge doesn't have to denote bitterness (Purple Passion asparagus, for example, is a super-sweet variety) but generally does in salad leaves. The Nine Palms Ranch Trial of Red Looseleaf Lettuce in 2008 backs this up. Results indicate that the darkest, most solidly purple leaves of cultivars Merlot, Redina and Red Oak Leaf, had the bitterest taste.
Of course, not everyone will agree that this is a good thing, but nowadays, when plant breeders are creating sweeter and sweeter crops, it's great to have something with a bit of bite to add to the milder tastes.
You'll also benefit from a wider variety of nutrients. Not only are dark-coloured lettuces, on the whole, higher in anti-oxidants, but also in Vitamin B6 and Vitamin K.
This is a controversial claim. Over at Vegplotting, a year of salad experimentation has revealed opposing opinions. Some people swear that slugs avoid red lettuces (as their Red Salad Bowl survived well), others think they actually prefer them (as their Lollo Rosso disappeared). In the past I've found that Red Salad Bowl tended to be second choice for slugs to its green counterpart.
It's likely that the level of bitterness has a bearing here and, unless it's a particularly bad year for pests, this in itself should deter them. If you suffer with slugs and snails, it's certainly worth experimenting with a couple of red varieties (maybe a lighter one and a darker one) and comparing them with your usual green ones.
The problem with shade
Unfortunately, purple varieties aren't for everyone. In shadier areas, leaves need to produce more chlorophyll to boost photosynthesis, so leaves are likely to be greener. For the rest of us, though, there are lots of good reasons to add colour to the veg patch.
By Helen Gazeley