Scientists expect that climate change will have a number of effects on our lives. As gardeners, one of the most important of these is a change in the arrival time of spring temperatures. Averaged globally, spring temperatures will start arriving very slightly sooner each year. However, at any individual location, the arrival of spring temperatures — compared to their expected arrival date — will be highly variable. Since spring is more likely to arrive early than late, make a plan to take advantage of this situation.

USDA Plant Hardiness Zones correspond roughly to the last frost dates in those areas. Mountainous regions will have later than expected frosts compared to flatlands. Check the internet to find your local average last date.

Most gardeners are well aware of their region’s average last frost date. Gardeners generally begin planting vegetables and annual flowers once this date has passed and the weather forecast indicates a continuance of above freezing temperatures. In northern gardens, the overall growing season is fairly short. For long-season crops, such as pumpkins, getting them started at the right time will ensure that they develop and reach maturity under favorable temperature conditions. In the south, vegetables needs to be planted promptly so that exceedingly hot temperatures don’t arrive until after the plants have flowered and set fruit. Most vegetables will not set fruit at temperatures over 95 °F (35 °C). As an example, the unprecedented heat in Texas in the spring of 2022 was disastrous to watermelon farmers. (It also did in my watermelons and limited my tomato crop.)

Sometimes it seems like spring has arrived early, but don’t interpret early warm temperatures as an indication that no future freezes are coming. Watch your local long-term weather forecast as your last frost date approaches.

As spring approaches, you should plan to plant when you normally do in relation to your average last frost date. As this time approaches, however, keep an eye on the long-range weather forecast. If your last frost date is still approaching, but the forecast does not include a freeze, consider planting at least some of your spring garden plants. Keep in mind, though, that nearly every year there are at least a few days of comparative warm weather near the end of winter. Every year, this entices gardeners everywhere to set out tomato transplants early only to have them freeze later. Recall that half of all final freezes occur after the average last frost date. So resist the urge to plant over three weeks early. That’s just asking for trouble. Planting early can reap big benefits, but it is also taking a risk. If your last frost date is three weeks away, and the forecast overnight lows are all above 36 °F (2 °C) or so, think about planting the vegetables that will thrive (or at least tolerate) those temperatures. For example, most peas will be fine when planted immediately after the soil thaws. Most beans, however, will do poorly if the soil temperature is lower than 50 °F (10 °C). Lima beans require even warmer soil temperatures and daytime air temperatures should be around 65 °C (18 °C).

Covering plants with cloches, row covers, or by growing them in mesh enclosures allows you to get a jump on the season. They will also keep out pests, but must be removed before temperatures get too high. (Cloche photo courtesy Wikipedia)

If you do plan to plant early, invest in some hoops and agricultural netting. This will allow you cover the rows if a light freeze is forecast. For individual large plants, a cloche can serve as a mini-greenhouse when cold weather threatens. If the plant is only a few inches tall, you can cut the bottom out of a gallon milk jug and use that as protection. Large plants, of course, can be planted indoors in small containers and moved outside when the weather is suitable. Even before climate change became an issue, gardeners had been doing this to raise transplants for spring planting.

You can plant garden plants indoors before it is warm enough for them to be outside. Take them outside on warm afternoons and bring them in at night. Or, keep them by a window that gets sun or under grow lamps.

Among the longer-term effects of climate change will be a shifting of agricultural zones towards the poles. North American gardeners on the southern edge of where growing a typical vegetable garden is possible will find it increasingly difficult, and eventually impossible, to grow anything other than a few locally-adapted plants. Gardeners on the current northern edge of vegetable gardening feasibility will start experiencing longer growing seasons and higher peak temperatures. The line on the map separating places that normally experience at least one hard freeze every year from places that do not usually experience freezing temperatures will creep northward. This line is currently in the very southern tips of Florida and Texas. Gardeners south of this line may experience more insect pests in the early spring as they won’t freeze in the winter. (And of course, some insects are adapted to survive freezing temperatures in the winter.) However, these conditions will not arrive overnight.

Conclusion

So, to make a long story short, expect the change in climate to proceed fairly slowly. (Or at least, it will seem slow from our perspective. On a geologic scale, it’s happening blindingly quickly.) But expect the variability in your local weather to become progressively more pronounced. As a gardener, expect that spring temperatures are more likely to arrive sooner than usual than later than usual. But realize that advance will be sporadic. Most years, your weather will likely stick close to the long-term averages. However, sometimes spring temperatures will arrive substantially earlier than expected. So plan for your usual gardening season, but be prepared to get a jump on your spring planting each year. If the indicators of an early spring are present, the risk of early planting should pay off more often than not.

Our planet is warming. Carbon dioxide (CO2) levels in our atmosphere have increased from under 300 ppm in the early 1900s to over 410 ppm today. This is mostly due to the burning of fossil fuels. Carbon dioxide acts as a greenhouse gas and the global mean temperature has risen 1.8 °F (1 .0 °C) in the past 20 years. This change in temperature has affected weather patterns all over the globe. On average, what we expect to see in the coming years is an earlier arrival of typical spring temperatures. Likewise, we expect longer and hotter summers, a later arrival of typical winter temperatures, and less predictable patterns of precipitation. The phrase “on average” is very important to understanding climate change. The average rate of climate change for any given location will be very slow, but the effects on local weather patterns will vary in intensity.

Carbon dioxide levels have been rising steadily in our atmosphere for decades. The average temperature at the earth’s surface is also trending upwards, but notice that it drops for short periods, too. Compared to the 20th Century as a whole, we are now between 0.8 and 1.0 °C warmer. (Figures from NOAA.)

In the coming years, your garden is likely to experience spring temperatures sooner than in past years. However, in some years, spring could come later. The chance of the latter is lower than the former, but far from zero. As we’ve seen in the previous decade, peak summer temperatures will usually be higher than usual. In a like manner, typical fall temperatures will remain a bit longer before falling.  In both cases, the opposite may occasionally be true. But those instances with be rare.

On average, this progression will be very slow. It will also vary with location. For example, regions near the poles are warming faster. In addition to changes in the seasonal temperatures, you may experience more or less rainfall than average, depending global weather patterns. You will also likely experience years during which your weather is relatively normal.

You can think of climate change this way. Let’s say you had a 6-sided die and you rolled it each spring. The number you got determined your weather. One and two meant cooler than usual weather. Three and four meant average temperatures. And five and six meant higher temperatures. In the past, with a fair die, those three outcomes would have occurred equally frequently in the long term. These days, the die would be loaded. It would be fixed so fives and sixes occurred slightly more often than they should. You could still roll a one through a four, though. You could even roll a series of ones, twos, threes, and fours. However, over time, you’d rack up more five and sixes than any of the other numbers. 

The ten hottest years of all time all occurred in the past 20 years. Nine of the past 10 decades have been warmer than the decade before them. 1940–1949 stands out as a comparatively hot decade. (Data from NOAA and NASA; tables courtesy of Wikipedia)

When it comes to planning your garden, the thing to realize is that the average change in your seasonal weather is going to be very slight. However, the variability in weather patterns will be higher than in past years. Most years, your usual planting schedule should work. However, in other years it might be substantially out of sync with the weather. As such, you may want to hedge your bets. Since temperature anomalies are more likely to be higher than lower, your best bet is to assume that you will experience normal temperatures during the gardening season, but prepare for warmer temperatures. In an upcoming post, I will explain how to do this.

In 2021, some parts of our planet experienced temperatures on par with their long-term average. Much of the Pacific Ocean, for example, was not hotter or colder than usual. And, some of Antartica was slightly colder than usual. However, most places on earth were warmer than usual — especially near the poles. (Figure from NOAA.)