Culling Plants, and Culling in Nature



Greenhouse with seedlings, 2009

    Not to pick up where another post left off--about first days landscaping--but every time I hear pounding rain on the stretched plastic of a greenhouse roof, I'm carried back to a January day in Petaluma, California where I logged my first day as a nursery worker.  My boss shepherded my into the little 48' x 24' hoop house, after we'd dashed from the barn.  Stepping inside the greenhouse, it sounded like being in a subway tunnel or on the tarmac of an airport next to an idling plane.  Greenhouse plastic amplifies the sound of rain, and the roar was alarming.  My assignment was to go through about 5,000 small seedling tubes of marsh plants and cull.  I had never heard that word.  So I watched as he did the first few.  Culling is a word I've since learned from this and other contexts, but I didn't know there was a real word for thinning the herd, so to speak.  It comes from Latin, where the root "coll-" meant 'to collect'.

  In nursery vernacular, 'culling' usually refers to the removal of dead plants from a large group, and the re-arrangment of that block of plants with only the remaining good, healthy ones.   Most good nurseries remove the worst living plants with the dead, as a way of upgrading the stock.  I culled pickleweed--Salicornia--for about 3 hours as water rose up through the grenhouse's gravel floor, as if the marsh plants were demanding their nursery environment seedling tubes were all sticking together and sticking to their holding trays, with roots tangled together.  It become a marsh right then and there.  It was hard on that first day to judge which live plants were weak or unhealthy enough to toss.  Something all nursery workers learn over time based on a few factors, boiled down to the idea of a standard for that nursery.  One easy guage you can try is: "Would you want to buy this one?", but that doesn't always work if someone isn't the type who buys plants at nurseries.



Deer in Ithaca suburb, 2011

    'Culling' has recently been used in relation to localized deer problems, basically meaning to reduce the total numbers by killing a significant percent of the deer herd in a given area.  I won't get into the right or wrong of that here, except to say that the nuances of the word are different from how it's use for plants in a nursery environment.  In a nursery, you wouldn't really refer to that as culling.  You might call that 'tossing plants' or 'pitching' them as a dear old friend from North Dakota used to say.  Pitching or tossing plants is something a nursery person hates to do, and only does when he has to admit he's grown too many of something to sell, and while it's a waste of money to throw plants away or compost them, it will cost more to care for them all.  But as you can see, the words are all a little different, even though they all refer in some way to reducing a quantity of a living organism for some reason.

   Nature culls its own, we know that.  Weak organisms are often easier prey; some are spared their eventual fate by the cruel pragmatism of the group: they may be eaten, killed, or abandoned to ensure they don't survive.  Evolutionary theorists argue about how animals of many kinds know to do this, or if in fact we are reading into their actions to see forethought or intent that's not really there.  But somehow they know.  It is perhaps biology's most fundamental idea--that organsisms struggle to pass along their genes to the next generation.  Barely hidden in this idea is another one: that organisms can recognize defective genes in their relatives. 

  In plants, we see a much more decorous society.  There is really no eating their own.  It's all left up to nature and her elements.  Yes, there is prey--lots of things eat plants.  But even the strongest most fit plants are stuck in place, there to be eaten by any animal willing to try.  Again the theorists: plants develop protection to defend against their sitting-duck problem.  Spines, thorns, toxic chemical compounds, rapid rejuvenation after being browsed--all these are ways plants have adapted to their fundamental flaw of being stationary.  Culling, then, comes at the hands of a more external cruelty.  As a plant you have to worry about a lot, but one thing you don't need to watch out for is being eaten by your parent, or being abandoned by one. 


Tsuga candensis, hemlock seedlings

    There's one more intriguing consideration with culling in the nursery environment.  With native plants we're growing them for their genetic diversity or their genetic specificity.  That means we either want a broad mix of genetics from within a given region to maximize appropriate options, or we want the genetics from a given habitat or location because we've decided that's what maximizes chance of survival in this particular case.  So we raise as many plants as we can, from our seed crop that meets those goals.  So should we cull the weak ones within such a subset of plants?  Once we've raised 1,000 spicebush for a project for example, should we cull out the 50 or so that are weakest and slowest to grow?  What if those end up being the most productive for seed output at maturity?  What if those happen to have a higher concentration of a compound that renders them more deer resistant?  What if the slower growers happen to be longer lived?  As a grower of native plants, we wrestle with these questions as they hide just below the surface of the more obvious decisions we make.  Some growers even argue you should save all the seed that doesn't germinate, because that reluctance--or recalcitrance as it's called in seed jargon--is precisely what these plants need in their gene pool to survive a changing climate.  After all, what better survival mechanism than not playing all your cards at once?  When you see a block of 200 columbine plants in 4" pots in the nursery, for example, not all 200 of those 200 germinated in the same tray.  We consolidate those out of the entire seed flat we sowed, based on the 200 that grew to a certain size with a certain vigor--a herd mentality, if you will.  You might call it culling, if you are feeling sorry for the ones we didn't pot up.  Or, you might call it reverse culling, or artificial selection, if you look at it from the perspective of the 200 we saved...it gets complicated.  In nature, it's Natural Selection--Darwin's key mechanism that acts on nature's raw, wild genetic stew.  But natural culling, or natural selection, plays out over time and space, massive amounts of both, which allows for the most possible factors to influence the culling.  When we do it in the nursery, it's kind of a sudden blow.  To keep the analogy warm, also consider the suddenness that comes with the 'culling' of deer.  In nature culling may happen suddenly on occasion but it is usually more of a process.

  Culling is one of those ideas that seem more and more nuanced the more you think of the concept.  Next time you buy plants, or read about 'culling' the deer population in a suburban neighborhood, think about what people really mean when they use the word...