Wednesday, 2 December 2009

Oh-oh! Oca allelopathy!

Is Oca Allelopathic?

Allelopathy is the ability of a plant to chemically suppress the growth of another.
I've just discovered the excellent Radix blog, which focuses on unusual root crops, and it strongly suggest that dried Oca foliage inhibits germination (of lettuce seed in the experiment).
This obviously is important when mixing and overlapping crops.

But it may not be all bad news. Managed carefully, decaying Oca foliage debris could be beneficial in ensuring a weed-free bed for any following crop, as is the case with several traditional green-manure crops. The real problem would be if the plants roots exuded suppressive chemicals while growing. This would make it highly unsuitable for bi-cropping. So far I have seen no evidence of this, but I will be watching out for it.

What I need to do is an experiment to test for this next growing season. Hmmm.

UPDATE: I have found evidence (here) that Oca roots produce an exudate which suppresses growth in competing plants. However, the chemicals are also beneficial - insecticidal, and pathogenicidal. At least some of the chemicals require UV light to be activated, (so effectiveness is restricted to the surface and first few mm of soil?).

UV light levels reaching the root exudates must be very low once the foliage has formed a canopy.

All interesting to know, but as is sometimes the case, science does not give a clear answer. Will Oca poison or protect its bicrop partner, or something between the two?
Well, it obviously does not seriously harm most crops, from the evidence of my own eyes, and the experience of generations of Andean Indians, who's traditional methods of cultivation include bi-cropping with maize, and mixed planting with other tuber crops.
It seems to still come down to 'suck it and see'.

UPDATE: Details and results of the allelopathy trial here.


  1. Ben Gabel of Real seeds tells me that he has noticed that oca has an inhibitory effect on the growth of crops planted the following season, although brassicas are apparently unaffected.

  2. Hi Rhizowen. Yes, I had heard that. I'm pondering how to test this out next season with different plant families. Easy if I had lots of space, but...

  3. interesting article you link to. But its a bit odd because they grew the oca from seed(!!!) and then grew it in 16h days, which I think is a bit long for tuberisation. So what they made in the experiment may not be what they make under tuberisation-conditions. Ben

  4. Yes, experiments involving photoreactive chemicals in a well-lit test-tube on the other side of the world will probably have a different result from what happens 100 mm underground in a UK allotment. Real life trials, preferably local, are the bottom line.

  5. Hello Ian,

    Oca is like other kinds of Oxalis not good to grow with beans.
    The chemical characteristics by Oca are to be investigated at the best one, if you with the Oxalis group is concerned. (If you understand me..)
    Best wishes Cordula

  6. Hi Cordula. I understand you well enough.
    It's a pity, because tall beans growing with oca would make a good combination in terms of space and light utilisation. Do you know of any recorded experiments which demonstrate a definite allelopathic effect on beans? Many traditional good/bad companion effects do not stand up to scientific tests, or are quite marginal.
    Another trial required?