Hello, I’m Yun, the founder of The Planting Key. Welcome to the page that lies at the very heart of my mission.
You deserve to start your gardening journey with information you can trust. In a world of generic advice, I’m committed to being transparent about where my data comes from and how I process it. This page is my promise of accuracy to you. Here, I will pull back the curtain and show you the exact, meticulous process I use to create the planting calendars you see throughout this site.
The Core Concept: Everything Revolves Around Frost
For most gardens in North America (specifically, USDA Zones 4-9), the entire planting schedule is determined by two critical dates:
- Average Last Spring Frost Date: The average date in spring when it is statistically safe from the last killing frost.
- Average First Fall Frost Date: The average date in fall when the first killing frost is expected.
These two dates define your “frost-free growing season.” My job is to help you work forwards, backward, and within this window to maximize your garden’s potential. My goal is to empower you with two distinct strategies—an Aggressive and a Safe approach—so you can choose the one that best fits your climate and your gardening style.
My 3-Step Calculation Process
Every date in my calendars is the result of a rigorous three-step process. I do not copy dates from other sources; I calculate each one to ensure internal consistency and accuracy based on my chosen methodology.
Step 1: Establishing Two Planting Baselines for Each USDA Zone
You may have noticed that even within a single USDA Zone, different towns can have different frost dates due to microclimates. To create a single, representative calendar for each zone that acknowledges this variance, my entire system is built upon two distinct planting baselines: an Aggressive baseline and a Safe baseline.
First, I select four populous and representative cities in that zone. This focus on major metropolitan areas ensures the resulting baseline is relevant to the largest number of gardeners. Then I use their frost date data to establish these two baselines for both spring and fall as follows:
The Aggressive Baseline: This is the calculated average of the frost dates from the representative cities. In spring, it’s the average last frost date; in fall, it’s the average first frost date. This baseline is for gardeners who want to maximize their growing season and are willing to protect plants from a potential late or early frost.
The Safe Baseline: This is based on the most conservative date from the representative cities. In spring, this is the single latest last frost date; in fall, it’s the single earliest first frost date. This baseline is for beginners or anyone who prioritizes plant survival, ensuring a virtually risk-free start.
For example, here is how I establish the two baselines for Zone 4. After selecting four populous and representative cities in this zone (Rochester, MN; Eau Claire, WI; Fargo, ND; and Bismarck, ND), I work with their established frost dates.
- The last spring frost dates for these cities are May 5th, Apr 30th, May 15th, and May 14th.
- Aggressive Baseline: The average is May 9th.
- Safe Baseline: The latest date is May 15th.
- The first fall frost dates for these cities are Oct 1st, Oct 7th, Sep 25th, and Sep 25th.
- Aggressive Baseline: The average is Sep 30th.
- Safe Baseline: The earliest date is Sep 25th.
You may notice the “Aggressive” fall baseline is later than the “Safe” one. This is intentional and is key to the strategy. The aggressive approach aims to maximize the harvest by extending the growing season as late as possible. This requires planting later in the summer and accepting the risk of an unexpected early frost, while the safe approach prioritizes harvesting well before any potential frost.
My calendars provide planting dates based on both baselines, giving you the power to choose.
Step 2: Researching Each Plant’s Unique “Timing Rule”
Next, I dive into the specific needs of each vegetable. Every plant has its own internal clock. I research this “timing rule” from the most reliable sources available, including:
- University Agricultural Extension Programs: Research-based guides from institutions at the forefront of horticulture.
- Reputable Seed Companies: Data from companies whose business depends on their customers’ success.
- Horticultural Research Papers.
This rule tells us when to plant in relation to the frost date. For example, a radish’s rule might be: “Direct sow 4-6 weeks before the last frost date.”
Step 3: Putting It All Together: Two Calculations for Every Plant
This is where the magic happens. I combine the plant’s timing rule with both of the baselines from Step 1.
Here’s a real example for a radish plant in USDA Zone 4:
The Radish’s Rule (from Step 2): Direct sow 4-6 weeks before the last frost date.
Calculation 1: The Aggressive Date
- My Zone 4 Aggressive Baseline (from Step 1): May 9th
- The Calculation: May 9th minus 6 weeks = March 28th. May 9th minus 4 weeks = April 11th.
- The Result: The Aggressive window for starting radish seeds is March 28th – April 11th.
Calculation 2: The Safe Date
- My Zone 4 Safe Baseline (from Step 1): May 15th
- The Calculation: May 15th minus 6 weeks = April 3rd. May 15th minus 4 weeks = April 17th.
- The Result: The Safe window for starting radish seeds is April 3rd – April 17th.
The fall dates are calculated using the same methodology.
You may notice a slight overlap between the Aggressive and Safe planting windows. This isn’t a mistake; it’s a valuable piece of information! The overlapping period represents the ‘sweet spot’ where the risk for early planting begins to significantly decrease, and the safest planting window opens. It’s a perfect time for gardeners to take a calculated risk or simply begin their cautious plan.
I repeat this exact dual-calculation process for every vegetable in every zone to create the comprehensive calendars on my site.
How to Find Your Precise Local Frost Date (Highly Recommended)
The calendars I provide are an excellent and safe starting point. However, for the most accurate timing possible for your specific garden, I strongly encourage you to find the frost dates for your local zip code. A garden on a hill may have a later frost date than one in a valley just a few miles away.
You can use the same trusted tool I use:
The Old Farmer’s Almanac Frost Date Calculator (by Zip Code)
Once you have your local frost date, you can compare it to my Aggressive and Safe baselines to decide which planting schedule is best for your specific microclimate.
My Commitment to You
My commitment is not just to give you data, but to give you understanding. I believe that teaching you how these dates are determined—and giving you the power to choose the strategy that’s right for you—is just as important as providing the dates themselves. This transparency is the foundation of everything I do.
For those interested in the raw data behind my calculations, I’ve made the full source data transparently available on a dedicated page. View Raw Data →
This dedication to meticulous, honest work is the very reason this website was created. You can learn more about my journey on the My Story page.
I hope this explanation gives you full confidence in the information you find here. Now, you have the knowledge not just to follow a calendar, but to truly understand and master it.
Now that you have a full understanding of the meticulous process behind my work, I invite you to put that knowledge to use. You now have the “when” and the “how” at your fingertips.