Sunday, September 6, 2015

The case of the missing seedlings

A little over a week ago, I planted seedlings of lettuce, kale, bok choy, collards, and mustard greens, happily anticipating meals later on this fall with these cool-weather leafy crops as star players. All went well at first; they survived their first night and full day in the ground. But then seedlings began disappearing. Time to put on my detective hat and get to work. What or who had motive and opportunity to kill my seedlings, and how might I prevent further occurrences of this dastardly crime?

Young seedlings can die from a number of different causes. Sometimes they just keel over, in the seedling flat or in the garden. Plant people call this damping-off. It’s caused by a fungus attacking the stem of very tiny seedlings and can lead to heavy losses in seedling flats. This doesn’t often cause the loss of seedlings transplanted into the garden, however, because most of my seedlings get pricked-out of the first flat and grown on in cell packs for a few weeks before transplanting into the garden. Even the younger seedlings I planted this time should have grown large enough to no longer be vulnerable to this particular criminal.

Predation by animals sometimes kills seedlings. I have witnessed rabbits and slugs eating seedlings in the past. A wide range of insects also attack seedlings. Since I have already had rabbits eat carrots and bush beans (a Black Turtle type, for dry beans) this year, and I thought it quite possible that they would enjoy eating lettuce as much as I do, I protected the portion of the bed where the lettuce is planted with Plantskydd, a repellant I purchased from Johnny’s when I discovered that rabbits were munching on the bush bean plants in late July. After I put down Plantskydd according to directions, I observed no further predation of the bean plants, which are now flowering. Does this mean Plantskydd repelled the rabbits? Well, maybe. But because I did not include a control (an area of bean plants that I left unprotected), I cannot conclude that the Plantskydd caused the lack of further predation, only that it is correlated with that observation. Distinguishing correlation from causation is one of the most pressing, and often difficult, problems in science, informal backyard garden science just as much as cutting edge research science. If I had protected a part of the bean bed with Plantskydd and left another part unprotected, and if I had then observed continued predation on the plants in the unprotected area but no further predation on the plants in the protected area, I would have some confidence in saying that the use of the repellant caused the lack of predation. As it is, however, the lack of further predation may have another cause. Perhaps the rabbits happened to find another food source they favored around the time I applied the repellant. And why, you might be asking, didn’t I include the control, unprotected plot? It was because there were so few bean plants left that I didn’t want them to be unprotected and it was too late to plant more beans in time for them to mature.

As every detective knows, in order to solve a crime, we must gather evidence. So I took a good look at the various seedlings, those which had been eaten and those which had not. The first thing that caught my eye was that no lettuce seedlings had been eaten. Hmm, interesting ... the area which had received the repellant had remained crime-free. But wait; so had part of the bed that hadn’t received it. Not only that, but none of the bok choy or mustard greens seedlings had been murdered, though it appeared that a couple of them were missing a leaf or two. Nearly all the attacks had been on collard and kale seedlings.

Now I had a pattern that might narrow down the list of suspects. Investigating, I found that in some cases all the leaves had been eaten off the seedlings, leaving only a tiny stem and maybe a leaf nearby. In others, a withered stem was all that was left. In a few cases, nothing was left. I considered what suspects might act in this way. In those cases where the stem had the leaves eaten off, it seemed likely that a leaf-eating predator was responsible, though I didn’t have enough evidence to finger the culprit. But it might not have been the only criminal. Nor could I think of a predator that would eat only kale and collards and not eat mustard greens or bok choy. Nevertheless, it seemed wise to put a repellant on all the remaining seedlings, one which would discourage insects and slugs as well as mammals. I chose diatomaceous earth (DE), a powder made from the fossilized shells of tiny sea creatures called diatoms, since it is allowed in organic production. As for how it works, let’s let Wikipedia weigh in: “The fine powder absorbs lipids from the waxy outer layer of insects' exoskeletons, causing them to dehydrate. Arthropods die as a result of the water pressure deficiency, based on Fick's law of diffusion. This also works against gastropods and is commonly employed in gardening to defeat slugs.” Mammals seem not to enjoy eating it either. So I dusted the leaves of all the remaining seedlings with DE after watering them well. Maybe, if I couldn’t catch the culprit, I could at least prevent further attacks.

Over the next several days, more collard and kale seedlings died. As of today, only three of twelve collard seedlings remain. No kale seedlings remain of the eight which I had planted. One bok choy seedling was also murdered, but all the rest of them, and the lettuce and mustard greens seedlings, remain and are growing. Hmm, the case was more complex than I had suspected. So I looked again, pondered ... and then I remembered something else. Of all the seedlings that I had planted, the kale and collard seedlings were the smallest. When I had planted the seedlings, the weather had been rather cool, but it has gotten much hotter since then (today’s high temperature at the official St. Louis station was 96F / 36C) and it has not rained since August 22. These are not conditions favored by any of these cool weather loving crops.

So we have hot, dry weather and a pattern of the smallest seedlings being far more likely to be murdered than larger seedlings. It looks like we now have two suspects, the weather and, as much as I hate to say it, me. For I was the one who had started the seedlings too late to obtain good-sized plants that would have the strength to resist difficult growing conditions. I think we can close this case.

But I still have a trick up my sleeve: some extra seedlings that I can plant in place of those that died. I always grow a few more seedlings than the garden plan calls for just for cases like this. In a few days the weather pattern is predicted to shift much cooler and it’s likely that rain will accompany the change. At that time, I’ll put in replacement seedlings. Whatever I get from them will be better than nothing. In the meantime, I’ll keep that detective hat nearby. You never know when I might need it.


  1. I've had birds eat seedlings too. It seems the attack vector possibilities are almost endless! :(

    1. Yes indeed. They do seem to stand a better chance of surviving the bigger they are when I put them in. Next summer I'll plan to start the seeds early enough to put out bigger seedlings. And I can always hope for better weather in August - not that I expect that, I just hope for it.

  2. Hi Claire,

    Beautifully written. That temperature would knock over cool climate crops here too. Plus you would have high humidity - I believe - because of the excessive rainfall. Those crops are germinating here right now and have been for a few weeks, so they really enjoy the cold weather rather than a harsh summer. Over summer, I grow them in a bed that only receives one hour of direct sunlight in the mid morning and they seem to do OK as long as the white cabbage moths don't spot them. Have you considered a shade arrangement over them until the get established?
    Cheers. Chris

    1. Making arrangements for shading the crops might help. I'll have to give some thought to how to do that. I know from whom I could purchase the shade cloth; it's figuring out how to best rig it up over the plants that is the part I need to ponder.

      Regarding your comment the last week, I'll be on the lookout for plans and ready-made housing as I work my way through the book on attracting pollinators. Mason bee housing and bee cocoons are readily available for purchase in the US. My first efforts with them were unsuccessful but future, better informed efforts might well meet with success.

      I haven't seen as many caterpillars or butterflies this year as I have in the past, for unknown reasons. Nor does the decline seem limited to my garden. I saw fewer butterflies and other pollinators at the Missouri Botanical Garden during a recent visit than I have in past years. They and others are promoting the growing of flowers that pollinators especially like. While that's a good response as far as it goes, it doesn't seem to address all of the possible causes of the decline.

  3. Lovevalerie from last week: good luck with your seed-sowing, and I will look forward to hearing how it works!