Sunday, 23 November 2014

Hidden statistics and parameters of a harvest

My parents owned a large garden, and with it came several old trees from an orchard.  Among them was a large walnut tree, and my childhood memories include collecting the nuts in September and October, and then leaving them in wooden trays to dry.  Then, over Christmas, my parents, my brother and I would take it in turns with the nut-crackers, breaking the shells and extracting the edible nuts.  I don't know whether Christmas was the optimum time as far as the quality of the nuts was concerned; it was clearly a good time for keeping children quiet!  With hindsight, my father must have selected this time by trial and error.  Like so many decision variables, it is one dimensional - too short a time and the nuts will not be dry, too long and they will have lost their flavour.  A second variable, which wasn't changed, was the ambient temperature of the room where they were stored. 

Commercial nut growers face similar decisions, but on a much larger scale.  The crop will not be uniform in moisture content, so the time from harvest to marketing the edible nuts will depend on the weather at harvest time.  But they will have sophisticated instruments for monitoring the crops; my father only had his eyes and the sense of touch, coupled with the possibility of testing samples at intervals.

All this came to mind as I was dealing with some harvest from my own garden.  We grow climbing French beans, of a variety which is no longer available commercially.  The seeds were given to me by a heritage grower, and the beans they produce are delicious, and freeze well.  So, to maintain a crop, we must keep seed from one year's crop for the next.  At the end of the season, we leave a few bean pods on the vines to ripen, and then collect the pods, take them indoors, dry them, and remove the seeds.  Today was the day for extracting the seeds.  Most of the pods had dried out, but not all, and some had started to grow a mould, other had been so wet when they were collected that they had begun to rot - not ideal.  And not all the seeds are viable.  So, I have a box of seeds to plant in 2015.  And I have faced the same decision with them as my father had with the walnuts - how long to allow them to dry out.  For me, it was about one month.  One day, one week, even two weeks, would not have been enough.  Two months would probably have been too much - the rot could have affected them all.  (One year I made the mistake of leaving the pods to dry near the central heating boiler, and the seeds dried out and were cooked.  Fortunately, I had kept some seed from the previous year.) 

Seed merchants have this problem too - though many plants for seed are grown in warmer, dryer climates than my back garden.  And different plants have different characteristics; just ask a biologist.

So, next time you buy a packet of seeds, stop and think of the statistical and biological research that has meant that your seeds will be viable.  All of that research, hidden behind a colourful picture on the seed packet.

That walnut tree no longer stands.  A few years after we came to Exeter, there was a phone call from my father; "We are all right", he said, "but the walnut tree blew down in the night, and we have no electricity because the tree fell on the cable".  Power was restored after a day or two, and my parents had the tree cut up.  As a reminder, we have a wooden bowl, made by a wood turner using part of the trunk.

Harvesting the nuts led me into trouble at school.  The outer casing of walnuts is a soft green shell, which turns black and oozes a liquid which stains.  As a youngster, I would often collect the nuts, give my hands a cursory wash, and catch the bus to school.  I would arrive with my hands marked with a yellowish stain; since nobody in the family smoked, I didn't know about nicotine stains, but people at school did, and accused me of being a secret smoker.  As collecting walnuts is not in many schoolchldren's experience, nor that of many schoolteachers, my explanations were regarded as a way of disguising my misdemeanours.  The stain did wash away, but walnut skins are still used to produce natural dyes.