Growing Apples through Unpredictable Spring Weather

Having lived in Wisconsin for a while now, I’ve become familiar with the infamous Wisconsin “false spring” - that period in late February or early March when the sun comes out and we get a hint of warm weather before, inevitably, the snow and cold return. This year, we had a few weeks in early March when temperatures climbed into the 50s and 60s. For many of us, myself included, it was a welcome break after a long winter. For apple growers, though, this early warm weather can bring devastating impacts for their orchards later in the season.

In fact, we’ve seen those devastating impacts as recently as last year. 2021 was one of the worst years for apple harvests statewide (and nationally) in decades. As Matt explained last fall in the Brix Project film Another Pivot,

As a cider maker this year I didn’t know how bad it was out there. I knew it would not be a good apple year, but I had heard of a couple orchards here and there that didn’t do too bad. So I was kind of going through the spring and summer thinking, it won’t be a good year but we’ll figure it out. And then apple season came, and I started calling the orchards we normally pick from, and orchard after orchard after orchard said, ‘I’m sorry, we just don’t have a good crop this year. I don’t know what we’re going to have for you.’ So as a cider maker, it means we have to do something different. It means another pivot.

In Another Pivot, we follow Matt as he deals with the fallout of last year’s low apple harvest, which meant scrambling to form new sourcing partnerships with orchards that were able to produce apples, as well as relying more heavily than usual on larger orchards. We also hear from local apple producers themselves, like Liz of Door Creek Orchard, who explained that last year was actually a part of a decade-long trend of climate-related events that have impacted her apple yield:

“The first year that we’ve had a major crop loss in our history - and we’ve owned this orchard since 1984 - was 2012 when we had 70 degrees for a week in February. We lost about 75% of our crop in 2012, it was actually a worse year than this year [2021]. Since then, it’s been almost every year that we’ve had some sort of freeze event, and it may not be an event where we lose a huge percentage of our crop, but if it’s not severe crop loss, it’s damage.”

What is it exactly about warm spring weather that makes apple growers and cider makers so worried?

In the film, Matt explained that warmer springs affect bloom time, or the time each spring when apple trees first put out flower buds. Bloom time varies every year because blossom development depends on a host of factors, including a certain amount of heat accumulation over time. For trees, sensing enough heat essentially marks the end of winter and onset of the growing season. These apple buds then grow into the blossoms that will be pollinated and form the fruiting body of the apple. But once buds are formed, they are highly susceptible to cold weather. If a frost hits after a tree has already developed its buds, that cold weather is likely to damage or destroy the apple crop.

After hearing these stories in the video, I wanted to understand more specifically how these spring weather patterns affect our apples. So I decided to dig into some data.

One way to estimate bloom time is through a metric called Growing Degree Days (GDD). GDD are a measure of heat accumulation beyond a certain minimum temperature threshold. Every plant has a different minimum temperature threshold at which it grows: for many apple varieties, the base threshold is 41°F. So below 41°F, apple tree development slows to zero. Temperatures exceeding 41°F signal the tree to grow - and the higher the temperature, the more GDD are accumulated. Certain stages of development (like forming new buds on trees) takes place after a set amount of degree days. For many varieties, apple blossoms will fully bloom at about 400 accumulated degree days. Of course, there are a lot of different factors that work together during apple development - but GDD give us a basic metric for understanding the relationship between weather and apple growth.

To show this visually, I plotted the maximum daily temperature for Mt Horeb in 2021 (in yellow). The horizontal green line marks the 41°F baseline temperature. Every time the maximum temperature exceeds that baseline means that more GDD accumulated. The vertical black line marks May 1, the date at which 400 GDD were accumulated and when, at least theoretically, apple buds should have begun to blossom. (Source for temperature and GDD data: NEWA).

Temperature and GDD data from NEWA, the Network for Environment and Weather Applications.

However, as we already know from the stories shared in Another Pivot, 2021 was a terrible year for apple harvests because of late spring frosts. Between May 8 - May 12, 2021, when many apple trees were already in full bloom, the daily minimum temperature (shown below in blue) in Mt Horeb dipped below freezing (the red line), ultimately destroying the apple blossoms.

A similar situation occurred in 2012, in which high daily temperatures (yellow) meant that the 400 GDD mark arrived on April 1, 2012 (shown in purple) which is far earlier than normal. When the minimum temperature (blue) dipped below freezing (red), those blossoms were killed. As Matt put it, “If you get that hard frost, that’s it. You might get a second bloom or maybe 10% of what you originally had, but there’s not much you can do about it.”

Of course, these graphs show a general representation of how temperature affects apple development. There is a lot of opportunity for nuance here: for example, an apple tree growing on a sunny south-facing slope will accumulate heat more quickly than a tree growing in a shaded valley. But when we hear stories about the dangers of the early warm spring, these graphs help show what’s going on.

Finally, I was curious about how 2012 and 2021 stacked up against other springs. Using the degree day calculator on the NEWA climate database, I looked at the daily GDD accumulation at the Dane County Regional Airport for every year starting in 1996 - the Mt Horeb database only included temperatures from 2018 onward, and I wanted a bigger picture than that. The following chart shows the date at which 400 GDD were accumulated every year.

Again, this chart is generalized: in reality, there is a lot of variation in bloom time depending on apple variety, other growing conditions, orchard location, etc. But the graph does show that bloom time in 2012 was actually a month earlier than typical. And what I find a little concerning here is how erratic spring temperatures were over the past few years. These unpredictable springs are yet another challenge that apple trees, and their growers, must adapt to.

As I write this blog on the last day of March, I’m watching snow fall outside my window. Personally, I’m ready for that 60 degree weather and sunshine again. But I also know that apple growers - and the folks at Brix Cider - are breathing a sigh of relief. At least this cold spring weather should help out our apples this year.

At the end of the day, even if spring temperatures stay cool, there are a number of other impacts that our changing climate can have on apples - and our local food system. This is the first blog post in a series about climate impacts on apples.

Jules Reynolds is a PhD Candidate at UW-Madison who is partnering with The Brix Project to study resilience in south-central Wisconsin’s food systems.