Wednesday, January 28, 2015

Plant rotation in the community garden

A caveat, of course, to begin.  This post (which is copied in here from the former blog at my website) focuses on a specific, small area of a particular community garden.  The topic of plant/crop rotation is often found in farming, not community gardening, literature.  And, there are those who would argue that this is way beyond overkill for our garden.  So, if you don't belong to this latter group, then forge ahead, intrepid reader...

Many rotation plans call for three year cycling: year 1 plant, years 2 and 3 fallow to that crop, year 4 plant.  I think this does a lot towards getting rid of pests for that specific crop/crop family.  I imagine that during year 1 the pest moves in and sets up housekeeping big time (if you're not using pesticides).  The pest 'overwinters' in any form from egg to adult and is ready to go come spring and your tender new plants.  But, you have wisely fooled that pest by not providing luxury accommodations in year 2...  Still, a hungry pest will generally make do with what it can find and might survive well enough to overwinter yet again albeit with a smaller population for the spring of year 3...  And now, the famine is serious, and the pest effectively dies out.  Or at least moves on to someone else's garden...  And so, in year 4, you can begin the cycle again.

The happy scenario I have imagined above (which theoretically gets you a good harvest of a particular crop family every 3 years) doesn't address the extent to which a crop has to be removed.  I don't know the answer because a lot of what I've read refers to crops and fields so the plant would be completely gone during the intervening years.  In the smaller garden space where crop diversification is the rule (especially with 20 or so gardeners on about an eighth of an acre), I suspect that the 'infestations' are not likely to be of the magnitude seen in acres of field planted in a single variety of one plant.

And, I don't know how far is far enough for a hungry pest.  Is 'further down the row a piece' or 'the row next door' far enough, or does there need to be an intervening row (or two or three) maybe with some specific plant the pest really hates in there?  I don't know but our experience with squash bugs this year does suggest that the pest situation can get out of control relatively quickly.  The bugs reproduced themselves with discouraging success, moved widely throughout the garden when their host plants were removed, and were willing to settle for cucumbers and melons when squash were unavailable.

Thus far I've focused on above-ground pests, largely because we've had so many this year.  But there are other benefits to plant rotation.  The rest of this post summarizes the information provided in an article I found online.  (Click on the word article in the previous sentence to go to the link - it should appear as a purple underlined text but you never know...)

This article cites (without references but I’m sure they can be found relatively easily) a number of studies of crop rotation suggesting that productivity falls off rapidly when the same crop is planted in successive years. One factor is the ‘nitrogen fixing’ (which is really about mycorrhizal fungi) and another factor is disruption of pests and other pathogens.  And the article mentions other things that just seem weird to me.  Like both snap beans and potatoes should be planted following corn to improve production.

Although the research is for farms, the article argues that even when pulling up plants in a home garden at the end of the season, we never can get all the roots so that pests and pathogens will remain.  Thus, even small scale ‘crops’ can benefit from rotation.  I've addressed above-ground pests extensively because you can see them and so you know what the problem is when your plant is unhappy.  I don't really know anything about the fungal diseases and viruses that always seem to mysteriously discolor, wilt and dry plants.  But the pathogen will often remain in the soil in an anthropomorphic way similar to the one I imagined for the bugs.  So, I think the same sort of temporal and spacial rotation scenario can be imagined.  The only difference I can think of would be that the fungal diseases really are going to be more localized than pests that can sort of 'vote with their feet' (i.e., move).

With the fungal diseases, though, the question of disposal of the diseased plant becomes important.  The material should be carefully removed and never, never, never put in the compost...  Think CDC and infectious diseases for your plants.

The article also provides an 8-crop rotation plan as well as a listing of nine ‘families’ to help rotate among different families and not merely different plants, some of which might have the same or similar susceptibilities.

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