First Published 9/30/2004
Things My Thermometer Never Told Me

Gardening in the fall often takes on the character of a game: do I or do I not cover my plants tonight? Guess right, and your frost-sensitive plants are protected from an overnight frost; guess wrong, and your plants are destroyed. But what about those mornings when you wake up to discover a stand of frost-burned plants, even though your thermometer indicates that the temperature never dropped below plus three?

Capricious Mercury

It’s been said that all a thermometer measures accurately is its own temperature. How true. Check the temperature reading on a thermometer hanging in the sun, then check one hanging in the shade; the sun-drenched thermometer will always read several degrees higher. Neither thermometer is wrong, but neither one can tell you what temperature your tomatoes are feeling at any given time.

The problem is that the typical household thermometer hangs at eye level outside the kitchen window – a distance of a good two metres or more above the canopy of your vegetable garden or pumpkin patch, and probably the same distance away horizontally. On a clear, calm night, the air near the ground surface can be much colder than the temperature even a couple of metres above the ground; if that thermometer reads plus two degrees C, the temperature at ground level could actually be zero.

Even in our greenhouse, we lower or raise our temperature sensors so they are just barely above the leaf canopy of whichever plants we’re currently growing. The plants don’t care that the temperature is 20 degrees at a person’s head height; all they feel is the 14 degrees C at floor level.

On cool, clear, calm nights, cold air, being denser and heavier than warm air, tends to hug the ground, sandwiched by warm air above and the earth below. Back when we were growing acres of pumpkins, squash and cucumbers, a cold night made it quite easy to tell where the low spots in our field were; they were the spots where the plants had the most frost damage. We quickly learned not to plant anything tender anywhere near these frost hollows.

For large commercial growers of tender fruits such as oranges and lemons, accumulation of cold, frosty air calls for drastic measures. Many growers install massive fans in their orchards to stir up the air, blending the warm air above with the cool air below. Others will use even bigger “fans,” flying helicopters over the orchards to push the warm air down to the fruit trees if a frost is imminent!

Warming Up Infinity

Strangely enough, it’s possible for leaves to be below freezing, while the air immediately surrounding the leaf is above freezing. It’s called radiational cooling; any object with a temperature above absolute zero (minus 273 degrees C) will radiate, i.e. release, heat. (However, it can also absorb heat from other objects at the same time, thank goodness, otherwise every object in the universe would quickly cool to absolute zero and freeze solid!) On a cold, clear night, plant leaves with an unobstructed view of the sky rapidly radiate heat out into the vastness of space.

You can see this effect for yourself simply by taking a look at the grass in your yard after a mild overnight frost. Grass directly beneath the canopy of trees will often be untouched by frost, while all other blades are glazed with a thin white layer of the cold stuff. The canopy of leaves simply prevents the infrared energy of the grass blades below from escaping into space, radiating it back to the lawn.

Betting on Bed Sheets

For many gardeners, the loss or gain of a few degrees here and there is of little consequence. But for those gardeners trying to grow huge, prize-winning pumpkins, every extra minute of growth counts. If the forecast is calling for an overnight low of plus five, it might be a good idea to keep those bed sheets handy.

First Published 9/23/2004
The Squashed Pumpkin

Saving seed is a time-honoured tradition followed by scores of gardeners. But sometimes, those carefully harvested seeds can yield some very unexpected results.

The Changeling

Recently, I was shown the fruit of a pumpkin plant – but a fruit you could never carve into a Jack o’ Lantern. It looked more like a flying saucer with scalloped edges. A local gardener had been saving the seeds of her favourite pumpkin variety every fall and sowing them in the spring; this year’s harvest yielded the usual crop of traditional pumpkins, plus the mystery fruit.

So what caused this aberration? A dose of cosmic rays? An exotic disease? A spontaneous mutation? Nope – just a little interspecies sex.

Kinky Cucurbits

Pumpkins are a member of the cucurbitaceae family, which includes cucumbers, honeydew watermelons, and gourds, among other plants. Cucurbits are indigenous to both the old and the new world; for example, pumpkins are from North America, while cucumbers are African. But the really interesting thing about the cucurbits is their unusual – and often shifting – gender roles.

Some cucurbit species produce perfect flowers; in other words, flowers that have both female and male parts and that are self-compatible (i.e., they can self-pollinate). Other cucurbits produce male and female flowers separately, but on the same plant; these species are called monoecious. Some have predominantly female flowers (gynoecious), and others have predominantly male flowers (andromonoecious). And to top it off, changes in the weather can change the sex ratio.

In Florida, for example, the hot nights cause cucumber plants to produce a higher percentage of male flowers, while in more northerly climes the cool nights encourage female flowering. Since it’s the female flowers that produce fruit, yields of northern cucumbers are often higher than those in the deep south.

Fertile Ground

What does all this have to do with the unusual pumpkin? Well, several members of the cucurbitae family are interfertile; that is, they can cross-pollinate and produce brand new fruit, fruit with traits from both parent plants. The female flowers of pumpkins have dozens of ovaries, and it takes dozens of pollen grains – one per ovary – to fully pollinate a flower. And those grains need not necessarily come from a member of the same species. Pumpkin plants are monoecious, and easily self-pollinate; but that doesn’t mean that they exclusively self-pollinate.

And therein lies the answer to the riddle of the mystery fruit. The owner mentioned that she had grown patty pan squash near her pumpkins. Patty pan squash is closely related to pumpkins; a pollen grain from a squash flower must have fertilized a pumpkin ovary, resulting in the unusual fruit. Perhaps you could call it a “pattykin.”

This phenomenon is more common than you might imagine; indeed, cross-pollination by bees is such a worry for commercial pumpkin growers that they segregate their crops; to avoid accidental crosses, no other cucurbits are grown within about a mile of the seed crop.

On the other hand, it’s not exactly a regular occurrence, either; home gardeners shouldn’t count on growing a crop of pattykins. If the pattykin seeds are saved and sown next spring, that seed could yield regular pumpkins, patty pan squash, or something comparable to the original pattykin, depending on chance and the other cucurbits grown in the garden.

Still, it’s fun to experiment, taking advantage of nature’s unpredictability. If you only see one pattykin in your lifetime, enjoy it; you may never see its kind again.

First Published 9/16/2004
Overwintering Trees

Here’s a frustrating scenario: in the fall, you and a neighbour each buy a fruit tree of the same variety, the same age, purchased from the same place. Yet when spring comes around, his tree is thriving while yours has succumbed to winter’s chill, despite the fact that as far as you can tell, you took better care of your tree than he did of his. Why will he be the one sitting down to enjoy a basket of freshly-picked fruit while you’re left with an expensive stick?

Give Me Shelter

Here’s the key: choosing a hardy variety is only the first step in getting a tree through the winter. Just because a tree is rated for the frigid climes of zone 2 doesn’t mean it’s indestructible. Even the toughest trees can use a little help enduring Canadian winters.

If you’re thinking of planting a fruit tree or two this fall, avoid planting in low spots, the so-called “frost hollows” where cold air accumulates. Trees in frost hollows suffer more injury than trees planted, for example, on hillsides, where cold air tends to drain away to lower altitudes. There can be a substantial difference in air temperature between valleys and hilltops; on a given winter evening, the temperature could very well be minus 30 on a hilltop and a full ten degrees cooler in a frost hollow. (The family farm was rather hilly, and we quickly learned not to plant frost-sensitive plants in the valleys; they were certain to freeze on cold nights.)

Wind is another enemy of fruit trees. Wind not only dehydrates trees, but cold winter gusts can penetrate deep into the tree’s tissues, causing injury. A windbreak, whether it be a building, a stand of older, taller trees, or a tall fence, will provide a certain degree of protection from wind chill. (As an added bonus, bees are more likely to visit trees that are sheltered from the wind, and more bees means better fruit set.) Of course, don’t get too carried away – if you provide too much of a windbreak, the tree won’t get enough sunlight to produce a good crop of fruit. And be sure that the windbreaks aren’t positioned so that they prevent cold air from flowing away from your fruit trees!

Bitter Harvest

Trees that set fruit heavily one year are often more prone to winter injury, because the energy reserves in the tree are diminished by the time winter arrives. Fruit drains plenty of carbohydrates from the branches, carbohydrates that not only fuel the tree, but serve as a kind of antifreeze. If the tree has set an extraordinarily large number of fruit (obviously not a problem this particular year!), selectively prune off some developing fruit early in the season to conserve branch carbohydrates, giving the tree a better chance to endure the winter cold.

Winding Down Gracefully

When the days shorten and cool in the late summer and early fall, trees start the process of adapting for the upcoming winter. Trees have the easiest time adapting when temperatures cool gradually, over a period of weeks. Of course, on the Prairies we’re usually not that lucky. Warm spells interspersed with cold snaps are all too common in the fall, and a tree’s mettle is really tested during these jittery starts to the winter season.

While there’s nothing we can do about the weather, we can ensure that we don’t make this hardening process any tougher than necessary on our trees. When the summer is winding down, don’t apply high nitrogen fertilizers or water to trees prior to their becoming dormant. Trees that are hardening off can de-harden if you feed them heavy doses of nitrogen. However, applying water and a slow-release fertilizer in late October is usually a pretty safe bet, and is in fact a good idea; the additional water and fertilizer will get the trees off to a good start the following spring.

There’s no guarantee that any given tree will survive the winter, even if you’ve bought hardy varieties, babied them all summer, and added all the protection you can. But every defence strategy you employ increases the odds that you’ll enjoy at least a few jars of jelly by next summer’s end.

First Published 9/9/2004
The Inverted Tomato

You’ve grown red tomatoes, yellow tomatoes, cherry tomatoes, bush tomatoes….but have you ever tried an upside down tomato?

This past summer, we had the opportunity to trial a hanging, cylindrical container with a large hole in the bottom – not for drainage, but rather as a space to transplant a tomato seedling. In this way, tomatoes can be grown upside down, dangling from the skies. The idea is that rather than having to stake the tomato, why not allow gravity to do the staking for you?

Inverted Expectations

Of course, the next logical question is: will a tomato grow upside-down? The answer is yes, and in fact it grows surprisingly well. Back in May, we transplanted one ‘Cluster Grande’ tomato into a standard 15 litre pot filled with soilless mixture and another ‘Cluster Grande’ into the upside-down container, filled with the same soilless mixture. The upside-down tomato was hooked to an arch in the greenhouse, while the standard container was placed a short distance away on the floor.

Both sat side by side in the greenhouse for three months and each received the same fertilizer treatment. I was somewhat surprised to discover that in the end both tomatoes had comparable yields and fruit quality. Apparently, living in a topsy-turvy world was no obstacle to this tomato.

Perhaps I shouldn’t have been surprised. Plants have the remarkable ability to grow in a manner that best allows it to capture sunlight, a phenomenon called phototropism; simply put, leaves grow towards light. You see it in your home whenever a plant leans toward a sunlit window, or when its leaves proliferate on the sunny side. Even turned upside-down, tomatoes express the same trait. The tomato we planted upside-down very quickly developed a growth habit resembling a fancy chandelier, with the stems arching up and out and the leaves twisting around to orient themselves face up. I also noticed that the fruit clusters formed at the same intervals on the stems as they did on the regularly planted tomato. As for the roots, they proliferated in the bottom six centimetres or so of the container, right near the plant’s base. This isn’t surprising, since roots would rather grow with gravity than against it.

The upside-down tomato is watered from the top of the planter, and the water drains out the same hole from which the tomato stem emerges. I thought that the dripping water might cause the tomato stems to rot, but the plant remained intact throughout the season, probably because the stems dried off rather quickly after irrigation.

Upside Downside

Now, lest you think that you should invert your tomato’s orientation, there are a number of caveats. First, the special container is expensive, far more expensive than simply staking your tomato. Secondly, planting up the container is time consuming and “a pain the rear end,” I heard one of our growers exclaim. Finally, you need a strong structure to hang the container from, particularly as the tomato grows larger and heavier.

I love it when adventurous gardeners look at their plants from a new angle and have the courage to try something new. But given its disadvantages and limited utility, I don’t think that this concept is about to turn the world of gardening upside-down.

First Published 9/2/2004
Late Bloomers

It seems like every fall, I come across a few instances of a rather strange phenomenon: apple trees with flowers, the blooms of spring appearing just as other trees are getting ready to shift their leaf colours to autumn hues of orange, red and yellow.

We all know that apples bloom in the spring. It’s only natural; you need flowers to set fruit, and several months of warm, sunny days are required for those fruits to mature. For apple trees, fall flowering is pointless; there’s not nearly enough time for fruit to develop before winter hits. Nonetheless, the flowers appear. Why?

Bud Battle

Flowering is a complex process, so it’s no wonder that it doesn’t always work out as planned. During the summer, an apple tree must first decide how to distribute its store of energy, and part of that decision includes choosing whether to produce buds that produce leaves, or buds that will produce flowers, both for next year’s growth. Producing leaf buds is a good choice, since more leaves means more energy for root and shoot growth. On the other hand, at least a few buds must be devoted to flower production, otherwise there will be no fruits and thus no seed to perpetuate the species.

Sometimes, one strategy will take precedence over another. For example, when light is limited, an apple tree will produce nothing but leaves, delaying flowers until light levels rise. Conversely, if a tree sustains a severe root or trunk injury, it may start flowering excessively in a last-ditch attempt to develop fruit and thereby spread its seed.

But these are extremes. During the course of a normal season, most apple trees will produce a lot of leaf buds, with a few flower buds thrown in, which makes sense because it takes between twenty and forty leaves to produce enough energy to support one good-sized apple.

Finding the Balance

To strike the right balance between flowers and leaves, the apple tree evaluates its environment and utilizes its hormones to regulate growth. One hormone cocktail encourages flowering, while another inhibits it.

At this time of year, flower and leaf buds that developed during the summer will produce next spring’s flowers and leaves, have matured and are toughening up to endure the winter cold. To prevent them from coming out of dormancy prematurely, the tree relies on a correct balance of plant hormones. The process is complex, but essentially bud dormancy versus bud growth is a battle between dormancy inducing hormones and growth promoting hormones. Once spring rolls around, growth inducing hormones override the dormancy inducing hormones, giving the buds licence to resume development. During the summer, the cycle is repeated. As fruit is forming, new flower and leaf buds form, mature and go dormant.

The system works well, but it’s not infallible. Sometimes, the summer buds haven’t fully developed and they teeter between blooming and remaining dormant. A slight “push” from a growth hormone can trigger flowering, which accounts for the pretty but functionally useless flowers of autumn. The buds inevitably die out when winter hits.

From what I’ve observed, it’s mostly flower buds that are affected. This phenomenon doesn’t seem to cause any long term harm to the tree, but the affected branch will have no flowers the following spring.

Hormone Trouble

The precise hormonal recipe that leads to late fall flowering hasn’t been confirmed. Periods of cool temperatures, bud immaturity, insect or disease attack, and even mutation are possible causes. Our best bet is to enjoy the late summer blooms; even if they are the result of a hormonal malfunction, they do seem to make spring seem just a little closer.