Thoughts on Microclimates for the Forest Garden


Once Spring arrives, if you’re like me, you naturally shed your mental resistance to winter cold. The cold that would have tickled in February stings the skin bitterly by April. But for most of us, knock on wood, the danger’s mostly psychological. 

(Naturally-occurring micrclimate in England, hosting tropical tree ferns in a temperate climate. Wikimedia)
But if you’re a fruit tree, (or fruit grower) there’s a real risk of loss as sensitive buds are prone to damage from a sudden, unexpected cold. 
One of our apricots is in just entering a sensitive stage, “first white,” as a polar vortex descends upon Michigan, threatening temperatures that could easily harm sensitive buds. At this stage, temps lower than 26F could cause a loss of fruit. This week, around Kalamazoo, we’ve seen temps as low as 19F. A real danger. 
And here’s where the risk-mitigation strategy of “microclimates” enters the story.
Because the factors of the physical environment have a significant impact on the actual temperatures of any given location. Concrete, for example, absorbs the heat of day and radiates it back out into the enviroment during the coldest hours of night. Buildings block the chilling winds that would carry this heat away. And so, the “urban heat island” effect means that while the temps reached 19F out at the airport, our low in the urban area was 24F. 

Taking advantage of these subtle design features like microclimates is a major part of Permaculture. 
Beyond the natural climate of our location, our site has its own designed microclimates, too. Since cold settles downward into valleys, the hillsides surrounding Kalamazoo create a “thermal belt” that typically buys us a few extra degrees of warmth and keeps damaging frosts at bay. 
(The view across the cold valley to the other side of Kalamazoo.)
But when it came to the most sensitive crops, such as apricots which break bud early and can easily be damaged by frosts, we stacked up redundant frost-resistance factors to help ensure we get fruit: 
1. Eastern Facing slopes are often recommended for frost-sensitive fruit crops. Our position on an eastern slope means we get the earliest rays of sun, breaking the coldest hours of the morning when temps elsewhere reach their daily minimum. But that barrier from Western sun also means that the daytime temps don’t reach their evening maximum in the afternoon, which delays the onset of spring at Lillie House by a week or two compared to other locations in the city. For other crops, a different aspect might provide a better microclimate. The best strategy is to play to the advantages of your site. 
(Home microclimates at Mother Earth News.) 
2. Risk mitigation around the home. We placed the apricot close to our home, in a sheltered location where it avoids the extremes of temperature. 

(Frost patterns early this morning. Here a cold “frost pocket” downhill.)

(uphill of the first picture, no frost to be seen.)

3. Frost-free zones around the home: we chose a naturally frost-resistant spot for our apricot, as is shown in this diagram from the Permaculture Designers’ Manual. 

(Home frost risks via the Designer’s Manual.)

4. Frost channels drain cold away from sensitive crops. In old-timey extension literature on apricots, it was advised to create “channels” of easy air flow that could allow cold air to drain away from sensitive crops and warmer air to accumulate uphill to create a protective blanket around sensitive crops. We took advantage of that strategy to provide protection to our gardens and slightly extend our season.

5. Planting microclimates. Windbreaks and hedgerows have been documented to create a milder microclimate around important growing areas, mitigating both the worst of winter cold and the hottest temps of summer. Again, this shows the natural advantage of climate-control created by the ancient cell-like structure of the “home garden” pattern discussed frequently on this blog:

(Via the study Nepalese Home Gardens)

5. Using forests. Forests create their own moderated microclimates, trapping heat in winter, sheltering tender plants, blocking out heat in the summer, trapping moisture and blocking dessicating winds. The relative placement of this apricot in our forest garden, as well as other sensitive plants, takes advantage of the “warm edge” effect observed at the southern edge of forests. 
And while slopes are nice both for moving heat around and moving water where you want it to go, a hill isn’t necessary for micrclimate design. On flatlands we can take advantage of other microclimate factors such as “thermal mass,” rocks, clay soils, water-absorbant soils, structures like homes, ponds, forests, trees, windbreaks, and so on. 
Alone, any one of these tools might buy you the few degrees or couple of weeks you need to protect your tender plants. But together, they really add up, providing protection through slightly different contexts and variables. With enough layers of protection, it’s even possible to successfully grow species a zone above your own growing zone. 
In the same way, we can “stack” design features that build fertility, protect against drought, fire or wind damage. Indeed, we can even use the same thinking to “design” more reslilient livelihoods, businesses or cities! 
And in the age of climate instability (and economic unpredictability) that kind of planning is more important than ever.  

Will all this be enough to save our apricot harvest from the polar vortex? We’ll see….
————————————————–
UPDATE: 
And the answer? YES! 2016 was both a year of major spring frosts and an incredible apricot harvest for us. We had no discernable damage to any buds on our apricot tree, or other sensitive trees, for that matter. 
And a question via email: can we still use cold sinks and frost channels on flat land? The answer is yes! 

As you can see from this image from the Permaculture Designer’s Manual, it is possible to create well-positioned frost pits, ponds and sink holes to move frosty air away from our sensitive crops. And keep in mind, the best strategy is always going to use stacked redundancies, getting as many frost-mitigating factors in as possible. These could include:
1. Frost channels and cold sinks to divert frost away. 
2. Thermal mass to collect and store heat near sensitive plants. 
3. Walls or other thermal barriers behind sensitive plants.
4. A 5-degree slope towards the south or east, depending on the desired effect.
5. Correct management of understory plants, which can help to create frost pockets or divert frost away from sensitive crops. 
6. Use of heat-generating plants, especially bulbs like daffodils and crocuses, which actually heat the soil in early spring and may have an effect on frost. 
7. Use of reflectors that might direct more morning radiant heat towards plants.
8. Hedgerows, windbreaks and forests that can shelter and create a moderated microclimate. 
9. Ponds and water bodies that can have multiple functions, acting as cold sinks, thermal mass, and reflection – all in one. 

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