You have spent time poring over blogs, Instagram, and seed catalogues to pick the perfect variety of seeds. They have finally arrived, and all of a sudden, this flower farming dream is becoming a reality. So exciting!

As you read the seed packet envelopes, confusion starts to set in. “Four weeks before last frost date, sow after last frost date, start eight weeks before last frost date.” Last frost date, last frost date, last frost date! How is it the same general direction for zinnias versus bachelor buttons?

If you’ve gardened in the past as a hobbyist, you typically look at the average last frost date. Once you begin professionally growing cut flowers for the market, you are no longer working with only one last frost date.

Still with me?

I quickly realized I didn’t fully grasp a lot of the common terminology on a seed packet (especially those written for professional growers) and how to apply that information to my crop planning. This information is invaluable once you understand what it means and how to use it.

I wanted to do a quick blog on breaking down these concepts and how to apply them. I am keeping it basic. You can get complicated, but for the sake of coming up with a crop plan and to give you a better understanding, I am keeping it simple by focusing on annuals. I will dive in a tad about perennial and biennial planting, but the truth is, once you take this information and apply it to annuals, you will find it easy to apply it to perennials/biennials.

Hardiness Zone: Tells You What Crops Can Be Overwintered

We see it on plant tags and seed packets at the nursery, but what does it mean?

I have been getting comments, emails, and messages from confused flower growers about how I am starting direct sowing seeds right now. They are in the same growing zone as me, and their ground is frozen solid.

It can get confusing, especially thanks to the seed packets designed for the hobbyist. They have a picture of the country showing hardiness zones that give instructions to seed sow based on your geographical location. No wonder people think hardiness zones tell you when to plant. Spoiler alert: hardiness zones don’t.

Quick Backstory on Hardiness Zones:

The hardiness zone was created by the USDA, and it was through bringing together a collection of average low temperatures in the different climates across the country (basically). They then took this data and assigned “hardiness zones.”

The lower the zone number the colder the average low temperatures are. The higher the zone number the warmer the average low temperatures are. You can click on this link to input your zip code and find your assigned hardiness zone.

The hardiness zones are not cemented in. They change, as weather patterns change. The USDA has actually reassigned zones many times before. Our area used to be classified as a zone 5! Some years, our zone 6b behaves more like a growing zone 5 when we have harsh winters, or in milder years, it behaves like a zone 7.

Update: the hardiness zones have recently been updated and have classified us to zone 7. Use your discretion on whether or not the updated zone for you is accurate. Personally, I disagree with the updated hardiness zone designated to us and think we are more on the cusp of Zone 5 and Zone 6. When in doubt, go at least half of a zone lower than your assigned hardiness zone to be on the safer side. For example, if your zone is classified as Zone 7a, approach your growth as if you are Zone 6b.

How, then, do we handle our growth? We prepare for the worst in case the winter is an exceptionally harsh one. We plan around a normal year (as a zone 6b). We take advantage of years that are more mild by bumping up our crop schedule. Last January, the field was frozen solid and covered in snow: there was no working that soil. It stayed that way through March. This year, it has been a milder winter, and we have been taking advantage of prepping rows, direct sowing, and planting out hardy crops under protection. It's the same zone, different years, and different weather patterns.

This information is valuable in telling you what you can overwinter. When a plant is hardy down to zone 6, I know I can grow that as a perennial or fall sow. If it is a more tender crop and it says hardy down to zone 9: it will not be overwintering in my area under my growing methods. For example, ranunculus are technically hardy down to zone 7. With minimal to no protection, these guys can be fall planted and overwintered. In our zone 6b, overwintering is possible but it takes some additional efforts and infrastructure (which we will get to in a bit).

Frost dates: going to tell you when

Here we are, circling back to the frost dates. The term “last frost” that seems to be printed on almost every single seed packet. If you’ve grown tomatoes you are familiar with the concept of frosts and consequences of them! The thing is, not every crop is as sensitive as the tropical tomato plant and its companions zinnias. To treat every flower as a tender crop is wrong. Also, some varieties are tough as seedlings and can handle those frosts but have been labeled as frost sensitive because at maturity, it can’t handle freezes. For example, sunflowers.

First… to better understand what the heck a freeze is.

Freeze: the temperature point where water freezes, which is thirty-two degrees F. Basic stuff, right? How does this effect plants? Plant cells are comprised mostly of water, which at the right temperatures (32F and below) over multiple hours is going to freeze the water, in certain plants, and thus cause damage to plant cells.

There are different levels of freezes (frosts). We learned to look at the frost dates from the perspective of it being more of a frost range.

Light frost: This is a light freeze, temperatures dipping below 37F down to 32F but only for a short amount of time, not enough to cause significant plant tissue damage. Hardy crops such as Larkspur and bells of ireland can handle light frosts, especially as younger plants.

Kill frost or hard frost: This is when a freeze occurs and the temperatures dips down to about 32F, for a significant amount of time. A kill frost will cause significant damage, even wiping out crops, and killing back foliage. Multiple kill frosts in a row can also cause the roots of a plant to die, if it freezes the ground. Once again, there are some varieties that can handle a hard frost but even the hardiest of the bunch will show some kind of frost damage such as droopiness or purple coloration but will most likely bounce back. A kill frost in the season is when it is most likely time to let go of the crop. A kill frost in the beginning of the season is a different story. If a hardy annual gets nipped by a hard frost, with a little dose of phosphorus it will most likely overcome the frost damage.

Two basic types of annuals: hardy and tender

Now that we know the type of freezes and temperatures to look out for, knowing more about the two main types of annuals will be needed to better crop plan and tell you the when to seed sow.

Hardy Annuals

If you are familiar with Lisa Ziegler’s “Cool Flowers” book and concept then you already know these tough blooms by that name. Basically, hardy annuals are varieties that can handle light frosts and sometimes even freezes with minimal to no protection. Hardy annuals are typically the spring flowers you see. Even in lower hardiness zones, most hardy annuals can handle being fall sown or planted and actually prefer it! The cold allows them to focus on a healthy root system that will lead to bigger and better plants come spring. Examples of hardy annuals: nigella, bachelor buttons, larkspur, strawflower and bells of ireland to name a few.

Tender Annuals

Tender annuals: these guys are really to blame for the confusion and our fear of planting out too soon. Tender annuals cannot handle even a light frost without some kind of protection. A light frost can kill back if not kill the plant, it will at the least delay and cause some purpling in the leaves. Planting out these guys before the soil is warm enough is of no benefit. Where pushing hardy crops early will lead to being rewarded, pushing tender crops early is a gamble. Tender annuals are crops that grow and bloom summer through the first fall frost. These guys not only like heat, they typically need the longer days of summer. Examples of tender annuals are: cosmos, zinnias, ageratum, hyacinth bean, nasturtiums, asters and marigolds.

Once you understand: the type of annual you are looking at starting and the levels of frost you can start putting that information into your frost date range.

There are two tools I use to make this part easier and this is where I officially begin to plan out my seed sowing dates, their corresponding successions and plant out dates.

Two important dates

Last Frost Date

This is the date where it is mostly likely the last day where the particular temperature occurs. When looking at last frost dates you want to look at them relative to the flower variety, starting with; is it a cold hardy crop or tender? This will tell you when you are going to plant out the crop. Typically last frost date comes down to a last winter frost date and a last spring frost date. There are many dates in between though! Am I confusing you yet? There are other variables that will also come into play when planning on planting out, which we will dive into in a moment. Basically a plant such as Larkspur can handle frost so looking at the last winter date is relative to that crop. In comparison, a cosmo cannot handle frosts so looking more at the last spring frost is more relative to that crop.

First Frost Date

First frost date: this is going to be the date where you get the first Fall frost. Hardier crops may be able to survive the first fall frost, if it is a light frost, whereas most tender crops (marigolds, zinnias, asters) will get wiped out. The first kill frost, when you see crops getting wiped out, will indicate the end of your growing season for these sensitive crops and when to transition to fall preparations.

You can visit this website, input your zip code, and get your area’s range of frost dates. On a personal note, from our note taking, we can see that the lows are a little extreme for our area. This is a tool to give you a launching point but really every growing operation is in it’s own microclimate. Heck, even some times parts of the growing patch have their own microclimate! Note taking is imperative!

Here’s an example of the table showing the frost ranges for our zip code. I know from my note taking that the lows and their projected likelihood is a bit extreme. I have planted many tender perennials in late April-May with much success. We just al… — Here’s an example of the table showing the frost ranges for our zip code. I know from my note taking that the lows and their projected likelihood is a bit extreme. I have planted many tender perennials in late April-May with much success. We just always prepare to cover just in case!

With the knowledge of the varying dates of average low temperatures for your area, you will take that relevant frost date for the crop you are looking to start and count backwards the number of weeks stated on the seed packet to get your first seed sowing date, your first succession. Then from there you can count forward to map out your other successions for that crop. granted, this does take knowing the basic growing needs of the crop you are planning. Before you get too carried away sowing zinnias through September…

You will additionally need to know your first frost date in Fall. This will tell you what the last succession of a crop you can plant. By using the days to maturity stated on the packet, you can count backwards from there, add a couple weeks and voila! Your last sow date for that crop. Here’s the link of a days from date calculator we use online, to save you from getting crossed eyed flipping through the calendar!

Some late spring low tunnels with some frost cloth to protect from unexpected frosts.

Other factors that will impact the when to seed sow

Soil temperature

It takes longer for the soil to freeze than the air. That’s why, if you can protect crops such as ranunculus from having their feet get frozen you can overwinter them successfully in colder climates.

A good indication as to when the soil temperature is ready is by looking at what volunteer seedlings are beginning to germinate. This is awesome in helping you confidently determine that it is time to sow that particular flower variety.

For example, on Instagram I had so many people wondering why I was starting stock in January. Well, in the field, under a row tunnel where our ranunculus are I could see stock germinating. Since it is a crop we prefer to have under a tunnel, I went ahead and started the seeds for transplant. Seedling volunteers also help to build data for future use.

What about if you questioning a variety you haven’t grown before, so therefore no volunteers?

I also like to check the soil temperature using a soil thermometer. A soil temperature of 60F is going to be the correct temperature for active root growth. Soil temperatures cooler than that is okay for some hardier crops. It will slow down root growth, they become “overwintering” temperatures which is why we prefer to plant out larger transplant with a larger (more robust) root system. The plant will slowly grow but overall is relatively dormant but once that soil heats up they will be ready to boom! The bigger the root system the bigger the boom and less chance of crop failure. Later in spring/early summer we can plant out crops relatively small
(think from 128s to 288s and 3/4” soil blocks).

Planting out dahlia tubers on a chilly May Day!

Once late April/early May comes around, I start checking the soil temperatures in the field more closely. This is going to tell me when it has become ideal to plant out dahlia tubers and some of my less resilient crops (not super frost sensitive but not really hardy either). Even if it feels chilly in the air, as long as the soil has been at the appropriate temperature for at least three days, it’s a fairly safe bet that it’s time to plant. Soil temperatures also effect seed germination. Some varieties need a cooler soil temperature from 55-65F where others like it warmer at a consistent 70F. Some varieties like warm days and cool night to germinate and grow optimally. This is all information that is stated on the growing information.

Daylight hours aka photoperiod

Photoperiod: love this fancy word. Not only do temperatures effect plant growth and bloom development but daylight hours do as well. Some flowers like the shorter days of fall and spring where other flowers need the long summer days. When you see “Harvest Groups” sometimes stated on the seed packet as Roman numerals “I, II, III.” A common flower to see this in is with Lisianthus. Even though all the varieties of the Lisianthus are sown and planted at the same time, the different groups will bloom at different times. They are telling you the needed amount of daylight versus nighttime hours. The Lisianthus varieties that are considered “Group I” will bloom earlier than Group II or Group III. The idea is to plant a range of harvest groups to have a longer harvest window. Other groups of flowers that this is commonly seen in are snapdragons, sunflowers and sweet peas. Typically not stated as harvest groups but it will tell you whether or not they are good for winter or early spring blooming or if they need the longer days provided by summer. This doesn’t necessarily mean that the corresponding temperatures that come with the daylight sensitivity work for all growing climates. Confusing right?

Let’s take sweet peas as an example. There are three basic varieties with different daylight requirements to bloom: elegance, mammoth and Spencer. Each of the varieties still need the same growing conditions. Elegance and mammoth are great for growing areas that have short springs or can grow during winter. Spencer varieties are better for the colder climates who have cool summers that provide the needed twelve hours of daylight but doesn’t come with high temperatures.

Snapdragons also have a range of varieties, where some are better for winter/spring blooming while others are better for summer/fall. For example, the Chantilly series is considered to be better for less daylight hours and cooler temperatures labeled as a Group I or Group II. Once daylight hours kick in, their stems become more fragile and shorter. This is when madame butterfly series can come in with a later blooming time being considered a Group III or Group IV. They can better handle heat and bloom under the longer days of summer. Then there are varieties that are considered solid performer under the short days or long days such as the Rocket series. No matter the group classification of the snapdragons, they all appreciate having a nice cool beginning to establishing their roots. Meaning, temperatures that cool flowers crave rather than those warm loving flowers.

Heirloom mums are incredibly photoperiod sensitive, which is why when you are looking at the King Mums catalog it lists bloom dates. Certain varieties need less daylight hours. This is why some are listed to bloom in late September while others are October or even into late November. If you are in an area that gets an early frost and don’t have a way to protect the mums, sticking with ones that bloom in September rather than November would be the safer bet. Or, if you are in an area doesn’t get a frost until late October/November or have a high tunnel, mums are a terrific way to extend your season.

Ranunculus, another photoperiod sensitive crop. If the heat doesn’t shut them down the long summer days surely will. Last year, we had a cold spring and needed ranunculus for a mid June event. Well, they started collapsing. We were in the low to mid seventies, had pretty chilly nights still but was the first year we planted them in full sun. So we popped shade cloth over them and were able to squeeze out a couple more weeks from them and have them for that event. They like the shorter days of spring.

Commercial greenhouses, can get really technical with messing with photoperiod to push crops to bloom out of their ideal season. Most of us are probably not going to go to the extreme but what it tells us is when the flower will optimally bloom. Most cold hardy varieties also perform best with less daylight hours. Summer crops, even if warm enough, will produce the best under the long summer days. A way to extend a crop’s harvest window is to get different groups that will bloom earlier than later.

On years where the soil is workable but we aren’t wanting to chance planting out seedlings we do winter sowing. This is a different concept outside of “cool flowers” method, in that we are not looking to overwinter hardy annual. Instead, we direct sow hardy annuals and allow nature to take it’s course. They will germinate once the growing conditions become ideal. Crops that we winter sow as examples are larkspur, bachelor buttons, nigella, orlaya and bells of ireland.

Season Extension Methods

Another factor that will impact when and what to seed sow will be your use of season extension methods. A season extension method is the use of low tunnels, double low tunnels, heated greenhouses or unheated hoop houses. Basically, it’s when we are giving some kind of protection to the plants from the freezes, moisture and winds. Bonus: it heats the soil temperature more giving you a bigger window for planting. Depending on which extension method you are using you can up your planting out schedule (thus seed sowing date) anywhere from two to four weeks earlier. If you have an unheated hoop house, you can also extend your season about two to four weeks after the first frost.

Applying this information

Your hardiness zone is going to tell you what you can safely overwinter in your climate based on those temperature averages. For perennials, you can plant safely those labeled for your zone or lower. Typically, biennials are fairly tough. Started as seedlings in the summer, planted out in the fall to harvest from the following spring.

This information is also helpful in determining what you can fall plant/sow and if your zone gets cold enough for certain crops or if you need to purchase pre-chilled bulbs or special varieties of peonies. Frost dates are going to start building your crop plans and seed sowing schedules, when to have plugs be delivered, etc. It is essentially the road map for your growing schedule. The frost dates are going to vary depending on your area and to hone down even further; your microsystem. Heck, even within your growing patch you will find pockets with individual characteristics such as being warmer or freezing easier!

Taking notes of these behaviors and the plant’s reaction is incredibly valuable and future You will thank you! It may seem tedious but trust me, it will give you a leg up on your growing. If I went off of what other people said (even local peeps!) I would be very behind in being able to harvest. Experiment, collect data and take notes.

I hope this post demystified when to seed start. The goal was to help give the basic understanding and foundation to begin to customize your personal growing schedule. Flower farming is not one size fits all!

Demystifying when to seed start for the flower farmer