The Importance of Controls


Fred is an old friend of mine who also gardens avidly.  He always produces a beautiful, productive garden envied by all who see it.  He seems to have a natural understanding of what plants want.  He loves eggplant, but growing them had always been a challenge for him in our rather cool climate.  One year he decided to try to experiment with alternative cultural methods to see if he could improve his yields with less effort.  Early in the summer I visited him and he excitedly explained the things he was trying.


"I've found a new variety this year, "Precious Precocious", that's supposed to produce fruit ten days earlier than the "Old Standby" I've been growing.  And I read in a magazine that growing plants on black plastic helps them produce more fruit sooner too.  I think it heats the soil better.  So I decided to experiment and do a little of my own research.  I mixed a lot of good rotted manure into the soil here and covered the eggplant patch with plastic mulch.  Then I put the transplants I started in the basement in holes in the plastic.  I put aluminum foil on the plastic beneath the plants that I read may reduce the insect problems too.  And I've been fertilizing with liquid fertilizer once a week.  I'm hoping this will really raise the production."


Now Fred is a nice guy, but he doesn't like to be told how to do things, especially by a know-it-all like me, so in spite of my misgivings about his methods, and because he had already started his "experiment", I said nothing but wished him good luck.


At the end of the season, an unusually sunny one with consistently warm temperatures, I saw him again and he reported his results.  Yes, the plastic and new variety seemed to help.  He had fruits almost two weeks earlier than ever and there were lots of them.  And he also noticed that there were no bugs.  "I didn't have to spray once for flea beetles or potato bugs", he said.  "I think I've found the ultimate way to grow these suckers."  I congratulated him for his success, but didn't tell him about my eggplants of "Old Standby" that had  fruited ten days earlier than usual and hadn't been bothered by bugs either.


The next year Fred followed his newly discovered procedure again, but it was a different season.  The weather was cool, quite cloudy and especially rainy that year.  Fred's plants didn't do so well that year.  The lack of sun didn't allow the soil to warm well.  The dampness promoted mildew and bugs were everywhere.  About halfway through the summer I visited Fred and found an exasperated, discouraged gardener.  Flea beetles had mercilessly attacked his eggplants followed hard on their heels by hoards of potato bugs.  What leaves were left were covered with powdery white mildew and he hadn't seen a single blossom on the scrawny, stressed plants.  "I can't understand it," Fred said.  "I did everything exactly like last year."


Fred was so sure he'd discovered the sure fire method of eggplant culture that first year, but what did he do wrong?  Fred's problem was lack of control.  He was actually trying to do multiple experiments but with no way of determining what effect each variable produced. 


In this case, he had made several changes to his previous method of cultivating eggplant.  First he had changed varieties to "Precious Precocious" without continuing to grow some of his previous variety "Old Standby".  He had no way of knowing for sure that any improvements were actually due to the new variety.  It could have been the weather that made the difference, or it may have been the extra manure he'd used.  And the lack of insect damage could have been due to any of a number of factors from a natural low in the population cycle, to an unusually high number of natural predators or the new variety or, as he had hoped, the aluminum foil.


Fred had no way of knowing what was really affecting his crop without something to compare each of his changes to.  He was missing a control. Without the control, his evaluation of his experiment was simply a guess and his conclusions were meaningless.


What could Fred have done to make his experiments yield useful information? 


First, Fred knew how "Old Standby" usually did for him.  If he had thought about it he would have realized that even "Old Standby" varied in its performance from year to year depending on weather and other local conditions.  To properly evaluate "Precious Precocious" he needed to grow some "Old Standby" plants for comparison.


Second, to determine the effect of the plastic mulch he needed to grow some plants with mulch and some without.  The ones without mulch would serve as the controls and provide a baseline for comparison. Using very simple statistical analysis Fred could actually measure the difference if he chose to.  Ideally he would have done that with each of the varieties.  He might have been surprised and found that "Old Standby" actually outperformed "Precious Precocious" when mulched.  His insect resistance experiment had similar problems.  I'll leave it to you to determine what changes Fred might implement to make those conclusions valid.

Finally, one year of evaluation, while it can provide a good indication of a new variety's performance, is not really sufficient to be confident in the results.  For a single person doing the experiment, multiple years of data will give more reliable results and conclusions.  A faster alternative would be to contact one or more friends, the more the better,  living some distance away in different climates. Ask each of them to do a replication of the planting.  Each planting should be the same as far as numbers, varieties and each of the other variables.  All the data can then be combined and you can arrive at a more dependable conclusion in a much shorter time.


Yes, it sounds like a lot of work and it is, but with planning and foresight it won't be hard.  The small plantings in a home garden will limit the tedium and the reward of doing the work the right way is a real confidence in your conclusions. You will be sure when you recommend a better variety or cultural method that you know what you're talking about.  And you experience much less of the frustration and disappointment Fred did.





Dan Sorensen   copyright© 2000