I have been thinking about marigolds lately, pondering if I should put some around my new vege garden. They are one of the first plants mentioned when people talk about companion planting, but what do they actually do? The more I learn about companion planting, the more I realise that a lot of the information out there is just recycled information about things that are ‘supposed’ to work. For all their good press, most people don’t actually know what marigolds do on a practical, New Zealand vege garden level. They smell bad, which deters pests, right? Maybe, not necessarily. One must also remember that much of the information out there is coming from different countries with different pests and different climatic conditions. Here is a small summary of what I’ve found out from a small research session.
Aphids & Whiteflies
Marigolds are said to deter aphids and whiteflies from tomato crops. Some sources imply that the smell of the marigold plants is hideous to these pests, while others state that the strong marigold smell masks the appealing smell of tomato plants from whitefly – http://www.cabdirect.org/abstracts/19971000929.html. Whichever way you look at it, this appears to have more application for tomatoes grown in greenhouses, as the greenhouse whitefly (Trialeurodes vaporariorum) seems to be the main problem. For outdoor tomato crops in the home garden, I’m not sure what proven effect marigolds have on whiteflies or aphids.
Feeding Parasitic Wasps
It has been suggested that marigolds provide nectar for wasps that parasitise pests such as stink bugs. Not a lot of research appears to have been done in this area so I haven’t got enough information to comment on this, particularly in relation to wasps and bugs that we actually have in New Zealand.
There are numerous mentions of marigolds, particularly African Marigolds (Tagetes erecta) and Stinking Roger (Tagetes minuta) killing weeds such as twitch grass, couch grass, bindweed and ground ivy. I haven’t yet found any conclusive research about this and I’m not hanging out for a weed super-cure, but some decent information on this would be nice. I suspect there’s not much to this one though, and that any research relates to field crops and thick plantings, which would have some smothering effect on weeds regardless of whether it was marigold or another plant used.
Nematodes are microscopic worm-like organisms that attack the roots of some plants. This is the one thing that marigolds are really good for. Marigolds have been shown to help suppress soil nematodes when mass planted due to compounds released by their roots. This webpage from The University of Florida – https://edis.ifas.ufl.edu/ng045 – explains a lot about the effect of Marigolds on soil nematodes, better than I can in a quick blog, and it gives many references for published papers as well. The main points I gleaned from this reading are:
1. French marigolds (Tagetes patula) are the best bet for helping to control a wide range of soil nematodes.
2. Planting a border of marigolds around your crops will have little or no effect on soil nematodes. They should be planted as a cover crop in your vege plot, at least 2 months (preferably longer) before you plant your veges, spaced less than 7 in (18cm) apart, and dug into the soil beforehand.
3. Planting a cover crop of marigolds won’t totally eliminate the nematode problem and it probably has to be done every year to keep having an effect.
A quick search on Google reveals that marigolds attract spider mites – eg, http://homeguides.sfgate.com/marigolds-tomatoes-spider-mites-51810.html. There are a lot of scientific papers focused on researching how to control spider mite on marigolds. If you have problems with spider mites on vegetable crops, don’t plant marigolds next to them as this will probably just create a bigger army. Slugs are also attracted to marigolds, but to many veges as well, so having marigolds probably won’t make much difference to your slug control plans.
Well, that was a lot to think about! A few things have become clear to me about companion planting. Sources should be checked for actual evidence, don’t just do things because someone said to. This can save you time and money on things that don’t work. Make sure you have a specific purpose for companion plants before you acquire them – what are you trying to control? If you want marigolds in the vege garden because they look pretty, that’s fine, but don’t treat them like impenetrable forts. I, for one, am currently more inclined to spend my money on other plants that will attract predatory insects with their flowers, such as sweet alyssum (Lobularia maritima) (traditional white cultivars), dill and coriander, and plants that feed bees, such as borage (Borago officinalis), sage (Salvia officinalis) and lavender. I am also very keen to read ‘Jackie French’s Guide to Companion Planting in Australia and New Zealand’ – http://www.jackiefrench.com/companion.html. Experiments and thoughts are welcome!
Notes from a Second Round of Research
One thing that didn’t come up in my initial research session, and has been commented on, was the effect of marigolds on white cabbage butterflies. This doesn’t appear to be a commonly suggested use for marigolds, however, some further research has shown that it has some merit. These two articles – http://www.slideshare.net/antonis3q/effect-of-intercropping-white-cabbage-with-french-marigold and http://agronomy.emu.ee/vol012/Metspalu.pdf – are particularly interesting. They suggest that planting rows of marigolds alongside cabbages can disrupt oviposition (egg-laying) of the small white butterfly (Pieris rapae), large white butterfly (Pieris brassicae) and diamondback moth (Plutella xylostella), whose larvae are also green caterpillars. It appears to be a small effect for the small white butterfly though, with better results for the large or great white butterfly, which only has a limited population in the Nelson-Tasman region at the moment – http://www.doc.govt.nz/Documents/conservation/threats-and-impacts/animal-pests/nelson-marlborough/great-white-butterfly-factsheet.pdf. The side issue is that the great white butterflies are very attracted to the flowers of French Marigolds (Tagetes patula), particularly yellow ones. Who wants to be feeding the adults even if they lay less eggs on your plants? I’m not sure if this is the same for our small white butterflies. For the small white butterfly, planting Pot Marigold (Calendula officinalis) next to cabbages sounds more effective than French Marigold.
My research took a tangent and I dug further into the control of white butterflies. I noted that frames covered in fine netting are said to be the most effective control. I am also keen to give the ‘imposter’ gig a go. You make butterfly shapes out of white plastic, attach them to the top of 30-40cm long wires and insert them around your cabbages. Being territorial creatures, this is supposed to scare the butterflies away. Apparently some white flowers have a similar effect. Now, if I could plant the whole vege garden in cabbages I could do some real testing. Pity I ain’t got the space left!