March comes in like a lion and out like a lamb. It is a time of transition. Here we are, in the middle of March, and our weather seems to change every hour, much less every day. Pouring rain opens to sweet sunshine, which is in its turn wiped away by thick dark clouds. It can be hard to know what is coming next, or how to plan. Today, we’ll be focusing on a faithful tool of farmers around the world: frost dates.

Frost dates are the two dates, specific to a region, when farmers and gardeners can expect the first frost of the autumn season and the last frost of winter. The first frost of the fall season is important to predict for all those varieties of summer produce: tomatoes, peppers, zucchini, summer squash. The goal is to have harvest completed before that first frost descends. The last frost, which is eagerly awaited as winter gives way to spring, is important for planning planting schedules. Tender spring and summer plants are not able to survive outdoors when the air and ground still dip below 32 degrees. Farmers and gardeners eagerly await the final frost date, to begin their work in earnest.

As many Hillsdale residents already know, weather can vary greatly by micro-climate. Hillsdale itself is at a higher elevation than some of its surrounding Portland neighborhoods, which means it can be colder in Hillsdale than it is in Lake Oswego, just a few miles away. Since frost dates are determined by historically averaged temperatures, it is worth noting that both the elevation and population density have a role to play in frost dates. Urban areas – sometimes also known as “heat islands” – warm up sooner than rural areas. The proximity of all those buildings makes a difference! In Portland that is best illustrated by the difference between the average last front date for downtown, as compared to somewhere more spread out like Hillsboro or Troutdale.

Before we had computers to track and analyze weather data, farmers relied on almanacs to provide them with their frost dates. The first almanac printed in the United States was released in 1792, during George Washington’s first term as president. Robert B. Thomas compiled this almanac, a descendant of which is still used today.

The Old Farmers’ Almanac’s official website says: “Based on his observations, Thomas used a complex series of natural cycles to devise a secret weather forecasting formula, which brought uncannily accurate results, traditionally said to be 80 percent accurate. (Even today, his formula is kept safely tucked away in a black tin box at the Almanac offices in Dublin, New Hampshire.)”

 That being said, there are almanacs in existence that date as far back as 1088! This first known almanac in Europe was created in Spain by Islamic astronomer Abu al-Zarqali. It is theorized by some that the word almanac itself is derived from the Spanish Arabic word al-manākh, meaning calendar.

As our climate changes around us, we will see less predictability in these frost dates. That will directly affect our beloved farmers, their crop, and their presence at market. Please have patience as some of our favorite vendors weather the weather and make their way back to Hillsdale!