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Red snapdragons and white cosmos growing under metal hoops in the Franklin & Co. flower field in Charlie Lake on September 16.

Sowing and timing

When to Start Cut Flower Seeds: Count Back From Your Plant-Out Date

Cory Franklin in her cosmos flowers

Cory Franklin. Grows cut flowers in zone 3 in Charlie Lake, BC, and built Stemwise so you don't have to research for 11 months like she did.

6 min read

Before I sowed a single seed, I had to build my crop plan on paper, run every variety through a planning calculator, work out how many seeds and trays I needed, then turn it all into a sowing calendar. On sowing day I'd have sheets of paper that would inevitably get wet, saturated and dirty, and I'd try my best to keep myself and my team organized. It was a lot of hoping and praying! And the two questions underneath it all were always the same: when do I start these seeds, and did I order the right amount of seeds? This post is all about the first one.

Why doesn't "count back from your last frost" work for every flower?

Because not every flower goes outside on your last frost date. Most seed charts count backward from your last frost, and that works beautifully for hot flowers like zinnias, which go out on or after it. In-between flowers go out close to your last frost date, so counting from it works for them too. Cool flowers are a different story.

The University of Idaho Extension says cool-season plants like pansies can go outside 2 to 5 weeks before the average last frost, while warm-season plants like zinnias go out 1 to 3 weeks after it. That's up to two months between the two groups. If you count back from your last frost date for a cool flower, your seedlings are still sitting on the shelf when they should already be in the bed.

Here's what that looks like on a calendar. Say your last frost date is May 15, and your cool flowers go out four weeks before it, around April 17. Count back from April 17, not May 15. Count from the wrong date and you start a whole month late. (If you need a refresher on cool, in-between and hot flowers, it's all in How to Plan a Cut Flower Garden Around Your Frost Dates.)

How many weeks before planting out should I start seeds?

It depends on the flower, and the range is wider than you'd think. South Dakota State University Extension says to start seeds anywhere from 4 to 12 weeks before they can safely go outside.

Ted Griess at the University of Nebraska Extension explains why the range is so wide: it depends on how quickly the seeds sprout, whether they like cool or warm growing conditions and how long they take to reach planting size. A lisianthus seedling needs about three months indoors to get big enough to plant. A zinnia is ready in just a few weeks.

Nebraska Extension also points out that some flower seeds need light to sprout, while others do better in the dark, so check each flower's needs before you cover the seed.

What happens if I start my seeds too early?

Starting early feels safer, and that's the trap. South Dakota State University Extension calls starting too early "the most-common mistake." Seedlings that sit inside too long get tall, weak and spindly, or rootbound in their little cells, and they struggle once they finally go outside.

Light makes it worse. The University of Idaho Extension says not enough light is "a major cause of transplant failure at home," and recommends keeping grow lights 2 to 4 inches above the seedlings for 12 to 16 hours a day. A sunny window usually isn't enough, especially in late winter.

So aim for the right start, with seedlings that are strong and ready on the day they go outside.

Which cut flower seeds should I sow straight into the bed?

Some flowers don't want to be started indoors at all. Larkspur is a great example. Nick Volesky and Melanie Stock at Utah State University Extension trialled larkspur as a cut flower in northern Utah (zone 6a) and wrote that direct sowing is the most common way to grow it, because starting it indoors "can introduce challenges with taproot development and transplant establishment." They sow it straight into the field at least 2 to 3 weeks before the last freeze. And timing paid off: earlier sowings gave up to 1.7 times more marketable stems.

Here's the but. A seed starting chart can make it look like everything gets started indoors, when some of the toughest cool flowers would rather go straight into cold spring soil.

How does seed starting change by zone?

In cold zones like mine, the season is short, so cool flowers get a head start indoors and go out in early spring, a few weeks before the last frost. Count back from that early plant-out date. I wrote down exactly when I start my seeds in Charlie Lake on The Farm Journal: When to Start Cut Flower Seeds in Zone 3.

In the middle zones, you have two windows. Cool flowers can be started for a spring plant-out, and some can be sown in fall instead. Penn State Extension times fall sowing of hardy annuals about six weeks before the first fall frost, which is a whole different date to count back from.

In warm zones, the calendar flips. Many cool flowers are planted in fall to bloom in spring, so you count back from your fall planting date. University of Florida IFAS Extension suggests starting seeds 4 to 8 weeks before your outdoor planting date and starting them in batches two weeks apart, so you always have fresh plants ready.

Same seeds, three completely different calendars. That's why one chart can't be right for everyone.

Cory's take

The research I've done says seed starting is a planting-out question in disguise. Once you know the day each flower goes outside, the sowing date almost works itself out.

My strong opinion? Write down your plant-out date for every flower before you buy a single tray. Everything else counts from it. When I started, I had to work out every one of these dates by hand, flower by flower, and it was a lot. That's one of the reasons I created Stemwise.

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Sources and further reading

Questions growers ask

When should I start cut flower seeds indoors?

Count back from the day each flower goes outside. South Dakota State University Extension says that's anywhere from 4 to 12 weeks before planting out, depending on the flower.

Is it better to start seeds early?

No. South Dakota State University Extension calls starting too early the most common mistake, because seedlings get tall, weak and rootbound before they can go outside.

Why are my seedlings tall and floppy?

Usually not enough light. The University of Idaho Extension recommends grow lights 2 to 4 inches above seedlings for 12 to 16 hours a day.

Should I start larkspur indoors?

Utah State University Extension says direct sowing is the most common way to grow larkspur as a cut flower, because starting it indoors can make its long taproot struggle.

I live somewhere warm. When do I start seeds?

Count back from your outdoor planting date, which for many cool flowers is in fall. University of Florida IFAS Extension suggests starting 4 to 8 weeks ahead, in batches two weeks apart.

Cory Franklin in her cosmos flowers

About the author

Cory Franklin

Cory Franklin grows cut flowers in zone 3 in northern BC.

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