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Smart Weeds

Smart Weeds


When I think about gardens, a happy little ditty comes into my mind.

    You can learn a lot of things from the flowers
    Especially in the month of June
    You can learn a lot of things from the flowers
    All on a golden afternoon.

These words come from a musical version of Alice in Wonderland. Both in the book and the theatrical version, the flowers are quite chatty with Alice. In fact, one might say some of them are downright nasty. I would not want to be among such unkind creatures. Nevertheless, although normal plants do not communicate verbally, we can still learn lots from garden greenery. No, no, not the plants that you want in your garden — the ones you don’t want, the weeds!

It never fails to amaze me how well designed weeds are for outsmarting people like me. A weed, to put it bluntly, is any plant growing where someone does not want it. Many of these plants grow in disturbed soil –such as a freshly cultivated flower bed. The ability to germinate in such a location is not as easy a task as one might suppose. There are a number of features these weeds must exhibit if they are to exploit disturbed sites when these become available.

The first necessity is that the seed is already available in the disturbed soil. In many cases these seeds have been lying dormant under overlying vegetation such as grass or trees. The seeds in the soil may have waited for years for the soil to be laid bare, and better still, to be turned over. Just how long some seed can remain still living in the soil was revealed through a most imaginative experiment set up in Michigan in 1879. Dr. W. J. Beal, a Professor of Botany at an agricultural college in East Lansing, collected seeds from 23 local plants. Subsets of fifty seeds from each species were mixed in moist sand and placed in each of twenty unstoppered bottles. These were buried upside down in a sandy hill. Every five years one of the bottles was dug up and an attempt was made to germinate the seed inside it. Dr. Beal supervised the work for the first thirty-five years and then others took over.

Of the 23 original species in the experiment, nine were still able to produce seedlings after fifty years. These included red-root pigweed, common ragweed, broad-leaved plantain, and black mustard among others. After eighty years had passed only three species were able to grow. These included evening primrose, curled dock and moth mullein (all popular weeds in Ontario). Then in 1970, in the ninetieth year, only moth mullein showed any life. Ten seedlings out of fifty appeared, most of which produced normal plants capable of yielding normal offspring.

So weed seeds can last a long time in the soil. This seemingly endless supply of viable seed in soil is the result of special design features which only certain plants possess. No matter how many seeds there are of a certain weed species in the soil, only a few of them germinate on any given occasion. Have you ever noticed that after each rain a fresh crop of similar weed seedlings appears in your garden. And you thought that you had eliminated all those pesky weeds for the season!! Well think again. If all these seeds had germinated on the first occasion, none would have been left to grow the next time, and the next and the next. Cultivated crops, on the other hand, germinate close to 100% with the appearance of favourable conditions.

Once the weeds start to grow, other special characteristics of these wonderful plants become apparent. As far as many plants are concerned, a minimum mature size must be achieved before flowering can begin. The whole point of flowering, of course, is to produce seeds for the next generation of plants. Many weeds, on the other hand, are able to flower and set seed even when they are very small. If the growing space is very limited and there is little rain water to be had, then a plant may flower when it is only a few centimetres high. Alternatively, if conditions are good, the sky is almost the limit when it comes to the potential of the plant for growth. For example, a mature plant of lamb’s quarters (native to Europe but found throughout Canada) may yield as few as four seeds when conditions are poor, but it may grow as high as 180 cm (6 feet) and yield 100,000 seeds when conditions are good, In addition, it does no good to cut down some of these weeds. Once flowering has begun the seed will set whether the plant is intact or not.

Every time I spy weeds flourishing where they are not wanted, I remember the wonderful design features of these organisms. Why are there still weed seeds in the soil after I have weeded umpteen times? It is because an individual plant has the capacity to produce seeds with a variety of germination requirements. Some seeds will start to grow at one temperature. Others need something different. Many weed seeds which germinate in the spring, prefer bright sunlight such as you find on the surface of disturbed soil, with warm daytime temperatures but quite cool nights. If there is little difference between the day and night time temperatures, these seeds will not start to grow. It is all so fascinating and such fun! My future objective is to find out how to banish the weeds from my flower garden. I must admit though that cultivated flowers are not half so interesting. So when your yard is full of weeds, just remember the wonderful design features of these pesky interlopers.

April 2001

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