Putting on colour for growth

Last updated 12:00 02/11/2012
CREATING A BUZZ: Bees often hang around white, blue and purple flowers in the garden.

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Have you ever noticed how different you feel in some rooms compared to others?

A room painted with red walls can make you feel energised or excited, while blue walls can promote calmness and relaxation.

The same can be said for plants and animals in the garden. Researchers at the United States Agricultural Research Service Centre in South Carolina have discovered that red plastic mulch - that is, red plastic sheeting - can energise your tomatoes. Red plastic mulch, also known as selective reflective mulch, laid under tomato plants can produce up to 20 per cent higher yields.

This is not a new study. I remember the idea of red plastic mulch being bandied about six to seven years ago, when I was editing Weekend Gardener magazine. However, at that stage the plastic mulch wasn't freely available. You can now get it freely online - although you may still need to look overseas to purchase it.

So how does it work? Well, just like you and me, phytochromes, which regulate plant growth and development, react differently to different colours, or more specifically, wavelengths of the light spectrum, particularly those in the red or far-red range.

Light reflected from the red mulch has a lower red to far-red ratio than normal sunlight. When the far-red light wavelengths reflect off the mulch and on to the tomato plants, the phytochromes kick into action and tell the plants to grow faster and produce more.

The reason for this is because unlike red wavelengths, far-red wavelengths are not photosynthetically active. In other words, they don't fuel plant growth. Instead, they enable plants to react to the environment.

Green leaves reflect far-red light, so if there are other plants nearby, a tomato plant will sense a high ratio of far-red to red light. In effect, it's sensing competition for sunlight, which prompts it to put on a growth spurt so that it can better compete for light.

By laying the red mulch, the researchers discovered that they could effectively trick the tomato plants into believing there was a lot of competition close by and induce faster growth.

The same researchers trialled red plastic mulch beneath strawberry plants, and found that the plants produced a higher yield, with larger berries and a higher sugar content. They also reported that blue plastic mulch under turnips produced a stronger flavour, whereas a green mulch produced a mild, almost sweet, flavour.

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In the Gisborne area, earlier this year, apple growers laid a white reflective mulch under their trees to help advance the fruit's maturity and give it a more even colour. They found that the mulch reflected more sunlight, and that the fruit lower on the tree ripened earlier.

In the research quoted above, however, turnips mulched with white plastic possessed the least flavour of all the turnips trialled.

Yellow attracts insects, so some farmers lay a strip of yellow plastic on, say, every fifth row of vegetables to entice insects away from the other rows. They can spray that row or leave it. Either way, the use of pesticides is lowered.

Coloured mulches may work better in different parts of the country. Research at the University of Florida found that red, green or blue plastic mulch produced no benefit to crops in that state. They concluded that coloured mulches worked better in cooler parts of the country.

Other research suggests that different colours affect different crops. A green mulch is apparently good for peppers and melons, while a blue one is good for eggplants.

Red in the garden is not much good for bees, though. Bees cannot see colours at the red end of the spectrum, and so cannot see red as a colour. They do, however, see colours at the other end of the spectrum - the blues, purples and greens. Unlike us, they also see ultraviolet light.

"This is often called ‘bee purple'," according to the International Bee Research Association.

"When you look at a white flower, the petals just look white. But when a bee looks at a white flower, it also sees lines that guide it down to the nectar - these lines reflect UV light and are invisible to us, but to the bee they are ‘bee purple'."

Which is why you often find bees buzzing around white, blue and purple flowers in the garden.

Blue and purple flowers also attract butterflies, and all the better if they're rich in nectar. When these overwintering insects emerge in spring, they need a ready source of it.

In spring, they're attracted to aubretia, rosemary, viburnum, philadelphus, lilac and honesty, among other plants. In summer, they'll go for cosmos, petunia, scabiosa, snapdragon, statice and verbena.

Buddleias are perhaps the best-known butterfly plant, and different varieties will flower over a long period.

Borage is a superb plant to have in your vegetable garden, as the blue flowers are extremely attractive to both bees and butterflies.

- © Fairfax NZ News

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