Why you should start seeds for next year’s flowers in September and October

Early fall is a funny time on a flower farm. In between harvesting buckets of summer blooms, we need to find time to prep beds… and start the seeds for plants we’ll be cutting from in the spring!

For the past few weeks, I’ve been sowing seeds like mad. My nursery area is slowly developing a carpet of soil blocks filled with baby snapdragons, stock, flowering cabbage, calendula, cerinthe, bells of Ireland, scabiosa, and other hardy annuals.*

Soil blocks on trays
Trays of soil blocks ready for seeding

Note to home gardeners—the time to start many of next year’s plants in a mild climate is actually RIGHT NOW.

Why is that?

Let’s rewind a little bit to explain the reasoning. If you’ve ever bought seeds, you’re familiar with the standard planting times on a seed packet, which revolve around first and last frost dates.

Know your average frost dates

In most of the United States and other areas with temperate climates, most plants go dormant or freeze during the winter cold, so you need to either get your hardy plants (the ones that can survive a frost) into the ground with enough time to get established before the first frost hits, or wait until things thaw again in the spring.

But how do you translate those instructions to a climate like Monterey’s (zone 10a, or Sunset zone 17), where we experience only a few scattered frosts each winter? Based on the packet directions alone, just about any time should be a good time to start cool-weather plants, but if you try planting most flowers or vegetables in December, they just won’t grow.

Why not? And what does that have to do with fall seed starting?

Enter: day length

You may be familiar with frost dates, but have you heard of the Persephone period? The term, coined by agricultural researcher and organic farmer Eliot Coleman, refers to the part of the year when there are fewer than 10 hours of daylight each day.**

Coleman observed that most plants stop growing almost completely during this period—at least, above ground. But while their stems and leaves may look the same from day to day, their roots are expanding, especially in milder climates.

Freshly planted seedlings

This means that once days reach that 10-hour mark again in the late winter, plants that have been in the ground since the fall have a nice, big root system and are ready to explode into healthy growth.

Fall-planted flowers and vegetables are ready to harvest much sooner in the spring than plants started in late winter.

Even if you start plants indoors in the winter and plant them outside when they’re the same size as the ones you started in the fall, they won’t grow as quickly because they won’t have had the extra month or two of time to get established and develop roots.

So now that you know why to start your plants in the fall, let’s see how to determine the right planting time for you.

Determining your fall planting dates

The first thing to do is determine the last day your location will receive 10 hours of daylight in the fall, which you can do easily with an online search. In Monterey, for example, November 23 is the last day in 2022 that will have 10 hours of daylight.

As a rule of thumb, seedlings need a month to establish themselves in the garden before this date, which means I should be planting mine out here at the farm by October 23.

Count backward to decide when to start seeds

Once you’ve determined your last planting date, you can refer back to the seed packet to check how long each variety needs to be ready for transplanting after you sow the seeds (usually about 4-6 weeks), which will tell you when you need to start the seeds.

It’s also worth noting that while it might be tempting to start seeds extra early, to give your plants have even more time to get established, it’s not a good idea to push the date too much.

That’s because hardy annuals prefer cooler weather to grow. So if you start them during the hottest days of August, they’ll generally be stressed, panic, and bolt, resulting in miniature flowers in December rather than tall, sturdy, abundant harvests in the spring and summer.

As a bonus, some flowers only bloom when days are relatively short. If you plant them in the spring, they probably won’t be mature until summer, and you’ll have to wait for flowers until the days start to get shorter again in the fall. But if you have them already established in your garden when spring rolls around, they’ll start blooming!

There you have it! If you’re planning a winter garden and haven’t gotten your cool weather seeds started yet, what are you waiting for?

* I highly recommend Cool Flowers by Lisa Mason Ziegler as a reference on hardy annual flowers, including a list of suitable varieties.

** Johnny’s Selected Seeds offers an excellent in-depth guide to winter growing for vegetables and flowers.

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