How Do I Know When To Start My Seeds?

Sorting out just when to tear open the seed packet and plant your new seeds can be frustrating. Germination times, days to maturity, frost dates, transplanting, direct seeding, all can be a little intimidating. Have no fear, we are here to help.

Frost dates

Unless you live in a part of the country where the weather is warm even in January, you probably have to deal with frost and freezing temperatures. 

Many seed packet instructions refer to the last frost date and will say something like, “sow into cells 4-6 weeks before the last frost”. There are other instructions, but for this post, we’ll stick to the task of planning out just when to tuck those little seeds into their spot.

While no last frost date is guaranteed, developing an accurate estimate is sure to save you some potential heartache. Planting out new seedlings too early can cause you to be SAD (seedlings all dead).  Knowledge of your estimated last frost date will help you to prepare seedlings that are ready and vigorous at planting time. 

Last or latest frost refers to the average latest date in spring of “light” freezing temperatures for your growing location. A light freeze is generally overnight or minimum temperature of 29-32℉, which will kill tender plants. Weather changes year to year and some years will have an earlier or later frost than others, so use these dates as a range to plan from, not a guarantee of safe planting conditions. 

As an example, we garden and farm in the cold state of Wisconsin. Sometimes we wonder if we will have frost every month of the year! Our last frost in spring is usually around Memorial Day, but sometimes it strays a bit into early June. 

Where to find the last frost dates for your location:

• The internet has many sources of information on frost dates, both early and late. Almanac.com and Plantmaps.com let you search by zip code. 
• Gardening books. Many good, informative gardening books will contain tables or maps of expected frost dates and planting schedules. Just make sure to reference a book or map for your location.
• County Extension offices. Many Extension offices have a horticulturist or agricultural specialist on staff who can assist with all of your growing questions. Some states also have a Master Gardener program. These folks are an excellent resource.
• Local Garden Club. These clubs can be challenging to find, but nowadays often have a social media page to aid them in keeping in touch and scheduling monthly meetings and member projects. Ask around, attend a meeting, and volunteer to help. Not only do these avid gardeners have a wealth of local information about frost dates, but also experience with local pest problems, wet and dry periods, soils, sources for plants, and more. You may even find yourself going home with a gift of several cuttings of new plants safely packaged on your passenger seat. 

Do the Math

Let’s use Madison, Wis. as an example. I have selected some Celosia and the information on the seed packet says transplanting is the recommended method, and to sow seeds into cells or containers 6-8 weeks before the last frost.

From the above sources, I determined that my latest frost date is about 28 April, so I’ll round that to the first of May to make things easy. 

I also found that my first frost date in fall is likely to be around 8 October, so to be a little cautious, I’ll consider the 01 October as my likely early frost date and consider anything past that to be a bonus. 

Counting backward on a calendar from 01 May gives me a date of 13 March if I use the middle time frame of seven weeks before the last frost. 

Sometime around that date, I’ll tuck those Celosia seeds into their cells following all the planting instructions on the packet, and talk nicely to them every day until they poke their little green selves up to say hello.

If I’ve done everything correctly, the little Celosia will be just the right size to harden off and plant out around my last frost date. I’ll check the short and long-term weather forecast, and cross my fingers. 

Short growing seasons and plants with longer times to maturity

If your growing season is short–ours is sometimes only 100 days long–you will likely want to  start many plants indoors when there is snow still on the ground as we do here at Stone’s Throw Flowers. 

While some crops can be direct-sown into the soil, we find that by the time the soil warms enough to do so, many crops do not have time to mature before the first cold nights of autumn. 

Maybe you need that plant to be ready (mature) a little earlier? As an example, the Celosia variety I have chosen takes 90 days to mature or be ready to harvest. It is not cold hardy, meaning that the first light frosts of fall are likely to kill it. Most of us buy our tomato plants as already large and vigorous seedlings, so we can enjoy tomatoes in the summer up until fall. 

Here at Stones Throw Flowers, our actual first likely frost in autumn falls around 10 September. Celosia that matures 90 days before that date would be around 12 June, just after our last frost of spring. No problem, right? 

But that means if we transplanted those cute little Celosia outside around 01 June, they would just be starting to mature and be ready to use as cut flowers right when they died. We want them to be blooming and useable for at least a month, so…

Back it up. In this case, we plan on those transplants being ready to go outside a little early (and maybe a little older, depending upon the plant)  and cover them with a caterpillar tunnel or other warming device. 

We also start and grow seedlings under artificial lights and will carry these plants in bigger cells a little longer to push that curve to the left. 

Knowing your last frost date range and when it is relatively safe to plant outside will help you plan when to start your seeds. Garden planning, seed starting schedules, and timetables for when plants will be mature all follow from this essential information. 

Happy planting!