Chill Hours (Baby It -Was- Cold Outside)

I realize that most people in northern areas are probably not terribly appreciative of this winter’s cold temperatures — at least at this point. I imagine they feel a bit like proper Victorian ladies sighing, “Hurry up and just finish please …”

Snowman3Maybe I can liberate your appreciation. The cold is not necessarily your enemy. I grew up in plant hardiness zone 3B: a place where hair froze in the morning while you waited for your bus (which was of course late). Where spit really can freeze before it hits the ground. But as cold as it got in the winter I could grow rhubarb, raspberries, lilacs and peonies.

As soon as I could I moved to the west coast. A rain forest. 9B. Heaven. I could grow anything.

Then I moved to Texas. (screechy record scratch sound effect)

I thought that as I moved closer to the equator I could grow more MORE MORE. What a greedy gobble gannet.

Gardening is ideal for helping one develop character. When I moved to the coast I learned joy and possibility. The move to Texas has been all about humility and limitations.

Temperature is a limiting feature but not in the way I usually assumed. I always saw things in terms of the plant’s cold hardiness number. I would ask: can this plant handle those bottom temperatures? How can I extend the growing season? I became expert at saving plants from frost and freezing temperatures.

When I moved south I learned that heat is also a limiting factor and not well described by the plant hardiness numbers. Everyone can tell you a plant’s cold hardiness but heat tolerance? Not so much.

And that was only the tip of the iceberg, as it were. I also had to consider chill hours.

Chill Hours

When I started to plan my garden I was shocked to discover that many of my favorite plants cannot grow here because it DOESN’T GET COLD ENOUGH. wtf. who knew.

Why would plants need or want cold temperatures? The best description that I could find of what is happening to plants during these chill hours comes from Martin Crawford’s Creating a Forest Garden. Basically, for plants growing in temperate climates there are three seasons: active growth, dormancy and preparation.

Active Growth is just what it sounds like. The temperature and light conditions are just right for plants to actively grow. Dormancy simply means the temperature and light conditions halt all growth. The period of Preparation is nice and mysterious. This is when temperature and light conditions are liminal and allow magical hidden things to happen. In the fall, cooler temperatures trigger protective actions as plants get ready for full dormancy. In early spring, warming temperatures trigger the pre-flower development and bud burst. If the job is rushed, the flowers might not have been prepared correctly leading to fewer and/or less healthy flowers. Few or poorly formed flowers will lead to less than desirable fruit.

There are some plants I will never be able to grow in Austin. And as climate change becomes ever more apparent choosing appropriate plants will become more difficult. Some local varieties that used to be considered well adapted suffered through the past 10 years because conditions were more like zone 9 than 8b. But it might be a mistake to replace everything with zone 9 varieties. Systems theory suggests that there will be some fluctuation and compensation during this time of change so temperatures will occasionally return to normal as they did this year. When temperatures do swing toward normal levels the borderline zone 9 plants can potentially die or try to grow & bloom too soon. Fevers and chills: evidence of the planet’s illness.

I found a helpful chill hour calculator online at Get Chill Hours. Chill hours can be calculated in a variety of ways, some aiming for greater accuracy than others.

Here is some information I found for Austin that might explain why last year I didn’t find a single pecan nut on the lawns. I wasn’t alone. A neighbor who gardens also noticed the lack of pecan nuts. Sure it was an off year but in the past even an off year meant a small harvest rather than a zero harvest. Now I think we did not get enough chill hours. This year there will be no excuse (fingers crossed.)

Model 2013-2014
This Winter
Last Winter
Below 45 degrees 1133 chill hours 524 chill hours
Between 32-45 Degrees 906 chill hours 493 chill hours

Comments and side conversations are welcome.

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