What is a weed?

07 May 2019


A germinating seedling



What is a weed? Can anyone give a complete definition? Because it appears to me that most gardeners it's "basically anything I don't want growing there"...


We received this question from SooYeah on The Naked Scientists Forum. Izzie Clarke asked Howard Griffiths from the University of Cambridge to dig out an answer as well as take on panellist Bill Colledge's own question.

Howard - Well, I think you've really answered your own question. But just to explain: beauty really is in the eye of the beholder. And if you're a naturalist and wandering through the countryside, many of the natural plants you see would be classed as weeds if you found them growing in a cultivated border. Equally so, you might see plants like invasive aliens, like Japanese knotweed, or a giant hogweed, or something like that, and you might think that's a weed of the countryside, we need to get rid of that one.

It really does depend where those particular plants are. I do remember as a child being told a joke, which I don't think is a very good joke, but it asked, how could you tell whether a plant is a weed or a border plant is: you pull them all up, and the ones that regrow are the weeds.

Izzie - Very true. Of course, they're really hard to get rid of. You can spend so much time pulling them out and thinking great, got that one done, and then give it a couple of days it's back again, so how can you actually get rid of them?

Howard - Well, I'm not a great believer in using chemicals and sprays although, of course, you can use those very selectively and very carefully if you paint them onto individual plants, and some of them will be taken down into the plant and kill the actual deep roots, that are the ones from which the plant will regrow. I'm a great believer in weeding, and hoeing, and using rather traditional methods of weed control.

Izzie - And speaking of annoying things in the garden…

As a bit of a follow-up - it seems to be on everyone’s mind at the moment - Emma has also got in touch to say

Q - How on earth do mealybugs/other bugs grow on plants? Where do they come from?

Are these those small green bugs that we see on plants, that we actually don’t want there?

Howard - Mealybugs are a kind of a scale insect. They're rather similar to aphids, they are a slightly different class of organisms, but they have similar penetrating mouthparts. We talked earlier about the water conducting system of plants, this is
the sugar conducting system of plants. So they dip into the phloem and they feed on that. And that's why they often exude droplets of honeydew, aphid honeydew, which you may hear of.

So where do they come from? Well, they overwinter in cracks and crevices, or in the soil, or they may be laid down as packets of little eggs from which the new instars will germinate once the cold weather has gone away.

Izzie - Are they a problem for plants?

Howard - They're a real problem for plants. Not only because they're feeding on some of the sugars that the plants are trying to supply to some of its growing areas, but they also transmit lots of diseases: lots of viruses are transmitted through these sap-sucking insects. And there's some very interesting science being done by my colleagues which shows how even the viruses change the behaviour of the insects in order to make them move on and move to another plant and so on.

Izzie - My goodness! Is there anything you can do to get rid of them?

Howard - Well, again, I'm all in favour of using warm soapy water to wash them off, or gently going along and squiging the blackfly on my broad beans with my fingers. I'm afraid it's a bit messy. But you can use safe sprays that are available from garden centres and so on.

Izzie - Bill, what would you like to ask?

Bill - When I was doing biology at school, my biology teacher told me a story and I just like to know if this is true. Scientists wanted to know what the composition of the sap was from the phloem and you mentioned the aphids tapping into it. And he said that what the scientists did was they took aphids and they chopped off effectively their head and left the bit that was in the sap, and they collected the droplets of sap coming out and they analysed it and that's how they did it. Is that correct?

Howard - There's some fascinating videos available on YouTube which can show scientists excising aphid stylets and collecting the sap as it drips out. The very clever trick that aphids have learnt to do is to is to plumb into the phloem without it blocking. If you artificially try to put a needle into the phloem, it will automatically coagulate and block and prevent the sap from flowing, but the aphids have learnt how to circumvent that problem.


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