Green Man Publishing

Climate

Gardening brings you into close personal contact with nature in all of her different guises and one of the most important aspects of nature that you have to deal with is climate. This has a significant impact on your garden, because it determines what kind of plants can be grown easily and which can’t.

Every climate has its advantages and disadvantages. My present garden has a long, 240+ day, fairly mild frost-free growing season, but for much of that time there is no rainfall at all. I am limited in what I can grow by the amount of water that is available during that time. The very mild winters can bring problems with lack of winter chill (some cold weather plants need a minimum amount of cold weather before they will happily flower). When I lived in Western Washington there was insufficient heat for my tomatoes and peppers to be really happy and they (and the potatoes) were sometimes affected by late blight. In my garden in Connecticut everything grew fantastically well during the warm humid summer months, but there were lots of insect pests and everything came to an abrupt standstill with the arrival of winter.

Some climates are more predictable than others, but the weather varies considerably from one year to the next. In my garden we have had winters with no significant frost at all and others with snow on the ground for several weeks. Rainfall has been as little as 30” or as much as 100”. Every region also has its extreme weather hazards, drought, hurricane, tornado, wildfire, flooding and more. These are mercifully rare however, as when they strike, the last thing you will be thinking about is your garden.

You can use various techniques to slightly alter the climate in the hopes of outwitting nature, but this is always a gamble and usually requires more work.

When we talk about the climate, we usually mean the macroclimate, which covers the whole region and gives us what we think of as the weather. There is also microclimate, which is the weather on a local scale and this can vary enormously in a small area, due to differences in topography. For example the tops of hills are colder than lowlands in the daytime because of the elevation. However they are sometimes warmer at night (and may have less frost) because cold air is heavier than warm air and so sinks and accumulates in low spots. Cities tend to be warmer than the rural areas around them, because of the extensive areas of thermal mass (concrete, dark colored buildings, roads and other paved areas). The leeward side a mountain (the rain shadow) tends to be significantly drier and sunnier than the windward side. See more on Microclimate in Site election.

Learn about your climate

It is helpful to learn about the climate in your neighborhood, as it will help you in planning the garden. You can often get useful information about your weather history from local weather websites (there is probably some weather buff living close to you who posts their own weather data online). You might also contact your local fire station, airport, newspaper, TV station or cooperative extension service office. Surprisingly few weather stations give much useful information that is relevant to gardeners, which is surprising as there are a lot of gardeners out there and the weather affects gardening more than it does most activities. On a wider scale I found the Western Regional Climate Center website to be helpful source of information (there is also an eastern one).

Generally the more local the data, the better, as weather can differ considerably in a few miles. I live only about 10 miles away from Santa Cruz, but I’m 2300 ft higher up and a whole zone colder in climate. A rain gauge and maximum/minimum thermometer will allow you to keep your own records.

Latitude

Latitude is your distance from the equator and has a major impact on your garden because it determines day length. The further north you go, the longer the days in summer and the shorter the days in winter. Long days mean more sunlight and  more growth and in the far north the very long days of summer can produce extraordinarily large vegetables. Conversely when the day length drops below 10 hours in winter there tends to be little appreciable plant growth.

Day length

In winter your gardening is more limited by day length than it is by temperature (you can raise the temperature fairly easily with cloches). The combination of shorter days and weaker sun, means there may be only an eighth as much solar energy around as in summer.

A common rule of thumb is that plants need a minimum of 10 hours of daylight per day to make significant growth. If they get less than this they grow very slowly, especially when combined with low temperatures. Bigger plants will often keep growing (rather slowly) at this time, but smaller ones will essentially come to a standstill. This is because growth is relative to size and a seedling has so little leaf area it just can’t produce very much food in the short time it has each day. A larger plant has a greater photosynthetic area and so may still produce a significant amount of new growth (enough to replace the leaves you remove in harvesting).

The dates when the day length drops below 10 hours in fall and when it goes above 10 hours in spring are important milestones in the garden year. Where I live there are about 4 weeks on either side of the winter solstice when we don’t get a lot of plant growth. Growth picks up noticeably as soon as we get above the magical 10 hours, and many hardy over-wintering plants start to bolt.

Day length also has another significant effect in the garden. Plants use the length of the period of darkness to determine when to produce flowers or when to start expanding their bulbs.

Altitude

For every 1000 feet of elevation gain, the temperature drops by roughly 4°F (this may be offset by other factors however). This is the main reason why my garden is several weeks behinds gardens in Santa Cruz which is only 10 miles away.

Temperature

You are no doubt already aware of the effects of temperature: most plants grow faster in warmer conditions, slow down as it gets cooler and stop growing altogether when it gets cold (below 45°F). This happens because the speed of a chemical reaction increases with temperature, so growth is more efficient in warmer weather. Of course it isn’t quite that simple, because each crop evolved to grow in a specific temperature range. A watermelon requires a lot higher temperature than spinach, which actually grows better at lower temperatures.

Precipitation

Human beings are creatures of the sun and rain is something most people don’t give a second thought to, except as an inconvenience. Until you have nursed plants though an extended drought, it’s hard to appreciate the near-miracle of water falling from the sky and landing right on your garden beds. The fact that much of it then sits in the soil until needed by the plants is another amazing thing. Yet another miracle is that when it falls as snow in the mountains it sits there for months before melting in summer and gradually running down to us.

As gardeners we should be more appreciative of rain, because it isw mor 3eimportant for good garden growth than almost anything else. In particular we should think about the rainwater that falls on our property and where it goes. We should try and ensure that any rainfall that lands on our garden has a chance to soak into the soil, rather than running away down storm drains. An inch of rain adds up to around 27000 gallons per acre, which at 1.5 cents per gallon adds up to around $400. I hesitate to quantify and put a price on nature, but I mention this to show how much she gives that we take for granted.

Rainfall can vary enormously from year to year, even in temperate climates. One year may endless rain and waterlogged soil, while another may bring endless sunny skies and parched soil.

An over-abundance of rain can be a mixed blessing in the vegetable garden. It is vital for plant growth of course and without it we can’t grow anything, but too much water becomes a problem. It can saturate the soil and drive out all of the air, cause erosion and leach out nutrients. If plant leaves stay wet for too long it can contribute to a variety of plant diseases and pests. Persistent heavy cloud cover also reduces the amount of sunlight reaching the plants.

Frost

Frost is something all gardeners outside of the tropics have to learn how to deal with. It occurs when the temperature drops below freezing point, causing any liquid water in the air to turn into ice. When this occurs tender plants will be badly damaged, or killed, as the water in their cells freezes. Hardy plants contain their own anti-freeze and will be undamaged by the same temperature.

In temperate climates frosts occur at both ends of the growing season, though exactly when they occur can vary by many weeks from year to year. The growing season is defined as the frost free period between the last killing frost of spring and the first killing frost of autumn. In areas with very short growing seasons frost can be an almost constant threat.

Frost occurs in two distinct forms, radiation frost and advective frost. Radiation frost occurs when the ground gradually radiates heat through the night until it drops to freezing point. This effect occurs over a wide area, but is most severe in places that are wide open to the sky. Advective frost is more localized and occurs when sinking cold air accumulates in low-lying valleys and hollows. These are known as frost pockets and you may see them white with frost, even though areas nearby are untouched. Wooded areas are less affected by frost because the trees slow down cold air movement, and limit heat loss through radiation.

The effects of frost are rarely uniform and they can assist you in locating the warmest and coldest areas of the garden. The areas where frost is thickest are the coldest, while areas with the lightest frost (or none at all) are the warmest. To find out if a low spot is a frost pocket just go outside after a very mild frost. The areas with frost on them will be the frost pockets.

Frost doesn’t kill plants uniformly. A sudden early cold spell in November might kill plants overnight, whereby a colder spell in January (when plants are more hardened) might not affect them at all. Wet soil exacerbates freezing by conducting heat more effectively. Deep snow acts as insulation and protects plants from severe cold. Cold air is heavy, so the higher parts of a tree may be unaffected by frost, even though lower branches are hard hit.

Frost isn’t always bad, in some situations it may even be beneficial for the gardener. It can kill insect pests and gives fruit trees their needed chill hours.  

The average last spring frost date

The last frost of spring is an important event in the gardening year, because it marks the start of the growing season. It is commonly used as a reference point as to when to start planting various crops (4 weeks before the last frost, or 2 weeks after the last frost).

It would take a crystal ball to accurately predict when the last spring frost is going to occur in any given year, but we can get a good idea of the probable date by looking at historical records. If you look at the dates when last frosts have occurred each year over an extended period, you can come up with an average date on which it occurs (this doesn’t take into account climate change, but it’s the best we can do).

The average date of last frost is just that, an average and 50% of last frosts will occur after this date and 50% will occur before it) The earlier before this date the more likely there will be frost and the further past it the less likely. Because this is only an average, it can sometimes seem rather disconnected from reality. For example my average last frost date is the 23rd March, but I have known the last frost to occur in late February and towards the end of April. Despite the ambiguously average nature of the average date, it is useful as a baseline to work from (just don’t be surprised if the last frost occur long before or after this date).

The variability of the last frost date presents you the gardener with a dilemma, as you know there is a 50% chance of a frost after that date. You could be conservative and wait a while longer before planting, to be better assured

of success. However this may waste valuable growing time, which can be important if spring (or the whole growing season) is short. It sometimes makes sense to plant early and be prepared to protect your plants if frost is predicted (or risk the possibility of frost damage).  

The significance of the last frost date depends upon what crop you are planting. It doesn’t have much relevance for the frost tolerant cool weather crops, such as the Brassicas, peas, spinach, lettuce or carrots. They can germinate in cool soil and a little frost won’t harm them at all. Even relatively tender crops like potatoes aren’t usually adversely affected (if frost threatens you can easily cover them, as they will be barely out of the ground). It is most important for the warm weather crops, such as tomatoes, peppers and beans that don’t grow well in cold soil and can be killed by frost. Once you have planted these crops you need to keep an eye on the weather and if frost is predicted you must protect them with straw mulch, frost blankets or other devices.

First fall frost date

The first killing frost in fall is also an important event because it will terminate the growth of any tender plants that aren’t protected. It marks the end of the “growing season”, though of course it doesn’t necessarily mean the end of your vegetable garden.

You can usually extend the growth of your tender plants at this time of year, by protecting them with frost blankets, mulch or cloches. Doing this can often earn you a few extra weeks of growing before cold weather finishes them off. When serious cold threatens your tomatoes and peppers, you should harvest any fruit that is nearly mature and ripen it indoors. This is always worthwhile as it can give you another few weeks of fruit.

Finding your first and last frost dates

I found the Western Regional Climate Center website to be very helpful (there is also an eastern one) for finding these dates.  My area is covered by the Northern California climate summaries map at http://www.wrcc.dri.edu/summary/climsmnca.html 

This map showed my closest weather station to be in Ben Lomond, so I clicked on that page. On the left of the page under Temperatures I found Spring ‘Freeze’ Probabilities. This brings up a graph showing the probability of the occurrence of various temperatures around freezing. Where the orange (32°F) line bisects 50% is the average last frost date, which for Ben Lomond I estimated to be about the 23rd of March.

The first fall frost date is also found under Temperatures as Fall ‘Freeze’ Probabilities, and has a similar orange (32 degree) line. Where this bisects 50% is the average first fall frost date. For Ben Lomond I estimate this to be about the 12th of November.

There is also a ‘Freeze Free” Probabilities page, which has a graph showing the length of your frost free season. For Ben Lomond I estimate this to be about 240 days.

Of course these dates are averages and so will vary from year to year (some years it will be earlier and some years it will be later). In my case I live about 2000 feet above Ben Lomond, which would affect my frost date, but Ben Lomond is in a valley where cold air collects, so I’m not sure how much (often I drive down into frost in the valley bottom).

The seasons

Unless you live in the tropics, you have to deal with the seasons, which dictate what you can plant and when you should plant it. They are caused by the tilting of the earth, which makes the days shorter or longer. The earth receives solar heat in the daytime and loses solar heat at night, so the proportion of day and night determines whether the earth gradually gains heat or loses heat day by day. During the long days and short nights of summer the land gradually gets warmer. In the winter the long nights and short days cause the land to gradually gets cooler. This gradual heat change explains why the warmest days are after the summer solstice and the coldest ones are after the winter solstice.

The tilting of the earth also affects the apparent strength of the sun. In summer the sun is warmer because it strikes the earth at a more direct angle and so the energy is concentrated on a smaller area. In winter it feels weaker because it strikes the earth at a more oblique angle, so its energy is spread over a larger area  (and I thought it was because we were further from the sun!) Of course we see this effect every day as the suns angle changes in the sky, it is weaker in the morning and evening than it is at midday.

In cold climates the growing season is often rigidly defined, as the ground may be frozen for months and all plant growth comes to a halt. In milder climates the differences are much less clear cut. My garden gets occasional frosts and snow, but much of the time it is so mild that many hardy crops keep on growing quite happily. In such places day length gives you a more useful definition of the growing season.

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