Mulching Potatoes With Straw

Mulching potato plants with straw is a method well known to us, in the world of permaculture. But the question of whether "to dig or not to dig" is not black and white. Certainly, conventional cultivation of potatoes requires a great deal of work (preparing the soil and visiting the potato patch to earth up the growing plants) but it also has its benefits: a beautiful earthed-up ridge promotes the proliferation of underground stems on which new tubers can grow and the risk of the tubers going green is reduced by protecting them from light. 

In the autumn of 2008, staff at the French gardening magazine, Les 4 Seasons du Jardin Bio (The 4 Seasons of the Organic Garden) were captivated by the enthusiasm of Bernard Patry (photo above) a gardener in the Charente-Maritime region, who had experimented successfully with mulching his potato plants. Intrigued by this approach, they asked their readers to participate in a France-and-a-bit-of-Belgium-wide comparison of growing potatoes under straw mulch against a control test of conventional earthing up. 

Mixed results
In general, their readers noted that potatoes under mulch were a bit slower (up to two weeks) to get going which, suggests Daniel Reichenbach, is probably because the covered ground warms up more slowly in the spring.  Slightly decreased plant growth was observed in most cases.

Weeds were less of a problem generally, although Yves Durand and Christine Leveroni—who both have heavy clay soil—found that bindweed proliferated under the mulch.  Some readers encountered a problem of increased slugs and even voles.

In terms of crop yield, a reduction, sometimes significant, was observed in over half the cases, both in the number of tubers and in the total weight per plant. But there were also many success stories. Pascal Chaplault reported: “I used a 20cm (8ins) mulch of straw immediately from planting; the plants took a bit longer to show but it saved me having to add mulch during the growing period. Production was excellent; I got a kilo of potatoes per plant ('Rosabella' variety), which were a nice size and a regular, rounded shape.  The conventionally grown potatoes gave a similar harvest but weren’t so pretty.”  “I picked potatoes ten days before their official maturity: it was a nice moment. I have never had such beautiful potatoes,” wrote Rodolph Grosléziat, “and none of them were damaged by the garden fork, which is not the case when I have to dig them out!”

For readers who enjoyed an equivalent yield between the two methods, the practice of mulching was strongly supported. The ease of harvest was also appreciated: Isabelle Laulon recounts: “In our sandy soil of the Landes, the potatoes were very clean, very pleasant to harvest and was a lovely surprise when we removed the straw.” Damien Meyrand adds: “With mulching, the potatoes sit on the surface of the earth and once the straw is removed, one only has to pull the plants by hand, a child could do it.”

The number of potatoes that turned green through being exposed to light, was more or less the same but, in a fifth of cases, was worse with straw.  However, as Damien Meyrand points out, “As with earthing up potatoes, the care taken in applying the mulch reduces the occurrence of greening.”

The ease of this method won it the support of the large majority of participants. The reduced need to water was another winner. Some suggested that mulching improved their soil, although drawing such a conclusion would probably need more than just one year of observation. 

Two-thirds of the gardeners who took part were motivated to continue with the mulching, feeling that the benefits outweighed the disadvantages.

What now?

A few suggestions to improve the method:

• Start with a well-loosened soil (particularly important with heavier soils).
• Plant potatoes 7-8cm (3ins) deep in the ground, i.e., don’t just lay them on the surface. With a better implantation, improved plant development can be expected.
• Mulch at planting, but not too much (as excessive thickness of mulch at this time was thought to be detrimental) then add more mulch during the growing season.  Bernard Patry, who inspired this survey, explains: “After the euphoria of my first experiment in 2008, the following year's results were less interesting.  My problem came from mulching too thickly at planting, using straw, leaves and grass cuttings. Some plants failed completely, others had trouble getting going and this handicap continued throughout the season in a lower growth. Next year I will just use straw for planting and then add leaves and grass cuttings while the potato is growing.”
• Yves Durand suggests working in squares (eg 2m x 5m) rather than rows: easier to mulch, less waste.
• The resource issue of straw was thought to be problematic (following a very dry summer in many regions of France this year, straw production has been greatly reduced).

More generally, consider the choice of mulch, and preferably use the gardens natural resources  (such as leaves, grass clippings, and the output from your garden shredder).  Perhaps even better suited to the purpose, these materials offer a more complete covering than straw, and as they decompose over the season they nourish the soil.  Francois Bonnin underlined the benefits of adding comfrey leaves, which are rich in potassium.

Rodolph Grosléziat compared the use of a mulch of grass (30cm/12ins) and a thick mulch of compost (20cm/8ins) (see photos 4/5 below). The potatoes mulched with compost started faster, with vigorous plants producing a significant crop, and the compost also benefits any succeeding crops which follow over the next four or five years - without the need for further fertilisation. A great way to think of your garden rotations.

Remy Bacher 

Originally published in the French organic gardening magazine, Les 4 Saisons du Jardin Bio, adapted and translated by Stuart Anderson. 

Les 4 Saisons du Jardin Bio (The 4 Seasons of the Organic Garden) is published by Terre Vivante, pioneers in organic gardening and practical ecology. The magazine, founded in 1980, now has 30,000 subscribers.
http://boutique.terrevivante.org/kiosque-presse/459-magazine-les-quatre-saisons.htm 

Further resources

Experimenting with potatoes

To dig or not to dig. How my allotment made the decision for me

Organic Gardening: The Natural No-dig Way

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