Step By Step Guide to Using our Seed Starting Calculator

Step By Step Guide to Using our Seed Starting Calculator

If you are like me, I am always thinking about when I can get my seeds started throughout the year. Whether or not you are an avid gardener or new to gardening, growing vegetables from seed is very rewarding, but to some a challenge. We have a great tool, the Seed Planting Calculator, that will help you get started, but you might also want to grab a calendar and the seed packets you want to plant.

First and Last Frost Dates

Before you get started with the calculator you will need to know the last frost date (for spring planting) and the first frost (for fall planting). If you don't know those dates we have a link to the National Climactic Data Center (NCDC) website. From here you can find out those dates. Here are the steps outlined on our website.

  1. Click Here (this takes you to the NCDC website)
  2. Under Product selection click on the down arrow next to Single-Station Products
  3. Select First/Last Dates
  4. Under Criteria select Min temp
  5. Under Pair results by: Select Calendar year
  6. Under Station/Area selection enter required station
  7. Scroll to the bottom and record the average dates for last (spring) and first frost (fall)
Frost date tool image

Here is a nice visual from the site. Choose the Single Station Products and from there select the First/Last Dates. The calendar year should be 2021 and the Min temp should be 32.

Next you will choose the station. You can put in your city, state or zip to pull up the stations that are closest to you. I do not have any stations close to my garden but I chose the Grass Valley 2 station for the closest one to our offices. A map will pop up and you can zoom in to see the stations near you. Once you find one that you want to use, just click on the push pin icon to select the station and then click Go. The chart with the max/min temps will come up. Scroll to the bottom of the page and use the average values. 

Enter these dates into the Seed Planting Calculator and click the Calculate button. We have divided the guide to direct seeding or starting ahead indoors. You will get a range of dates to get started.

Determining the Planting Date Using a Calendar

You can also do the same thing if you use a calendar and the information listed on the back of your seed pack. You will however, need to know your last and first frost dates, though. So as an example for the Cherokee Purple Tomato, the planting and growing instructions say to sow seeds 6-8 weeks before the last frost. So if your last frost date is 4/19 then count backwards from that date 8 weeks. Those seeds should be started around February 22. If you start the seeds around that date, the seedlings will be ready to transplant out into the garden around the last frost date.

You should have some rowcover on hand, just in case you have any low temperatures around the time you set your transplants out. 

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Maria, the spring planting dates will give you either a direct sow icon or a start indoors icon. So the dates will be reflective of what you should do, either start indoors and set out when the seedlings are big enough and the temperatures are also warm enough. Or you can direct seed things like radish, in those date ranges that are given.


Melissa, you need to look at the information on the seed packet or information posted on our website to see the suggested germination temperature for the seed you want to start. It really varies from seed to seed. Depending on how cold your greenhouse is right now, you will want to keep warm weather seeds, like tomatoes, under a dome to keep them warm enough. Seeds like spinach, can tolerate colder temps in your greenhouse.


I love this tool, thank you!
Can you please confirm that the spring planting dates that are “starts indoors” are indeed the date to start those seeds indoors and not the date to plant the seeds started indoors outside?
Thank you!


Do seeds have to maintain temperature above 65 to germinate? I have a small greenhouse I want to start seeds in, will that be successful? Or should I start seeds indoors and once I see beginning growth then get them in the greenhouse?

Melissa Kinsey

What a useful tool! Your instructions made the process to get great information so easy. Thank you! I can’t wait to start sowing seeds.


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