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Monday, 11 April 2016

Plants that work together

By Helen Roberts

As we roll into spring, gardeners eagerly collect packets of flower and vegetable seed to plant in their gardens. I have my disorganised pile of seeds ready and waiting, nestled in their respective packets, overflowing out of a tin stored on my kitchen dresser. Random seed that I have gleaned from gardens over the course of the year can be found in bags and random pockets.

Companion planting.
Photo credit: Brian Pettinger courtesy of Flickr.
My vegetable garden at home is minuscule and consists of a number of pots in the front and rear garden. I don’t own a greenhouse but use an area in my parent’s and beg ground from a friend to grow larger vegetables. Space is limited and therefore my crop is valuable. I don’t want attacks by cabbage white or carrot fly, I need to grow at different levels to maximise space and grow a wide range of small crops to give a varied meal. This has made me think more about what I can do to increase my crop productivity. I never use pesticides or herbicides in my garden and my resolve in this has been reinforced by recent press coverage over the importance of urban gardens for pollinators. I actually quite like cabbage white butterflies - my children do anyway - and they would be horrified if they knew I sprayed to rid the garden of them. Instead, I have decided to use the art of companion planting.

What is companion planting?

Companion planting is an age-old agricultural technique used for centuries across the world. It involves the idea of planting crops that are mutually beneficial to each other in order to increase productivity.

Why should gardeners companion plant?

Companion planting has a very wide number of benefits and uses:

1. Mix it up

It is best not to put all of your eggs in one basket when growing crops - grow a mix of crops rather than a monoculture so if one crop fails you have other crops as a fall back option. Moreover, a mix of crops will make it more difficult for pests to find their host plants, a hypothesis known as the ‘disruptive-crop hypothesis’.

2. Plants that give a helping hand

The other advantage is, if like me your garden is space constrained, you can plant on different levels. For example, a tall crop (such as corn) can provide a trellis support for a climbing crop (such as beans) and a ground crop (such as squash) provides shade and discourages weeds. You get three crops in a small amount of space and the companion plants provide physical advantages for the other crops.

3. Provide a home

Some plants provide shelter and shade for other plants; for example, the planting of corn can provide shade for lettuce or spinach (although most vegetable planting in this country requires as much sun as possible). Companion plants can also provide refuges for many beneficial insects.

4. Pest control

Marigolds are supposed to help keep aphids away from tomato
plants. Photo credit: Ruth Hartnup (on Flickr).
Companion planting is also supposedly helpful in pest suppression through the release of repellent chemicals. Numerous companion plants also attract beneficial insects, such as ladybirds and lacewings, which predate on crop pests and many of these insects also act as important pollinators. Some plant species are used as ‘trap-crops’ drawing the attention of pests away from the crop and acting as sacrificial plants. Companion plants also provide a visual distraction to pests. In a monoculture, pests move easily from one plant to another, but companion plants break up this assault.

Folklore or scientific fact?

It is hard to know whether some traditional planting combinations that have endured the decades are based on any sound evidence of benefits. Certainly there is plenty of information about companion planting in the popular press and gardening books, but most is not backed up by any rigorous scientific trials in the peer reviewed literature. In academic literature, experimental results investigating the use of certain companion plants are varied as to their effectiveness. Here are some examples that I have come across:

Odorous onions

I learned from my maternal grandfather to plant Allium species in amongst my carrot rows to help deter the pesky carrot fly, Psila rosae. This annoying pest feeds not only on the roots of carrots but also on other crops too including parsnip, celery, parsley and celeriac. The small 9 mm creamy white maggots cause scarring of the tap roots making them inedible and more prone to secondary rots. This year I plan to plant my carrots in amongst garlic chives, a plant similar to the onion chive and which also produces pretty white edible flowers.

It is generally thought that aromatic species deter pests by exuding repellent chemicals. Most relevant to the carrot-allium combination was a study that looked at mixed cropping of onion and carrots and the effects on pests of these crops. They found that there were reduced attacks by carrot fly compared to monocultures of both crops and that when the plants were planted together in high densities this also reduced plant pests and increased predators of carrot fly eggs. Stan Finch and his colleagues from Horticulture Research International in Warwick found no evidence that odours from aromatic plants repel or deter crop pests, suggesting that reduced attacks on host plants were a result of other mechanisms, such as simple disruptive effect.

Get a sniff of marigolds

I am not a big fan of Tagetes species, more commonly known as marigolds. They just remind me of old fashioned bedding plants in staid Victorian parks, but if they ward off pests in the vegetable plot then I am willing to use them. Tagetes sp. exude a strong odour that is apparently not pleasant for plant pests. They are supposedly useful to plant with tomatoes to ward off greenfly but I have yet to find any scientific literature to support this planting combination. Again, Finch and his associates specifically found that it was not the odour of the marigolds that repelled the pests, but simply the fact that they acted as a diversion for the pests.

Setting a trap

I often grow pots of nasturtium (Tropaeolum majus) in my garden. I like the look of the flowers and especially the leaves, using them as salad garnishes and the seed to make capers. This plant is said to help attract black fly, aphids and other pests away from host plants acting as a crop trap and functioning as a sacrificial plant. There are mixed results as to whether nasturtiums really do this. One year in my garden I grew cabbage and nasturtium together and both crops were smothered in cabbage white caterpillars. The butterflies definitely did not show a preference for the Nasturtium and the caterpillars eventually obliterated both plants. Certainly there is not a wide range of recent academic literature investigating the use of nasturtiums as a companion plant. However, according to the Royal Horticultural Society (RHS), this plant is beneficial to pollinators and as I can also eat it, I plan to trial a variety known as Blue Pepe, known for its distinctive bluish leaves and bright red flowers.

The scientific literature on many companion plant combinations is mixed, but what is clear is that there are no disadvantages to using companion planting. The physical advantages of companion planting will be evident even if the other benefits are less obvious, and many companion plants can be found in the RHS's 'Perfect for Pollinators plant list'. Even if you don’t get increased crop productivity, then the garden will be diverse and look beautiful too.

Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.


Sources:

Associate Professor Jane Mt. Pleasant 

Uvah, I. I. I. and Coaker, T. H.  Effect of mixed cropping on some insect pests of carrots and onions. Entomologia Experimentalis et Applicata 1984; 36: 159-167

Finch, S., Billiald, H. and Collier, R. H. Companion planting-do aromatic plants disrupt host-plant finding by the cabbage root fly and the onion fly more effectively than non-aromatic plants? Entomologia Experimentalis et Applicata 2003; 109: 183-195