Starting flowers from seed is an essential part to a successful cut flower farm, however, it can be very challenging at times!
When I started my farm, I had tons of experience starting vegetables from seed, and I naively thought starting flowers would be just as easy. Well, let me tell you…it was not! I was shocked at how much different flower seeds were than veggies.
Each flower type has its own unique germination preferences: some need light to germinate, some will germinate in darkness, some like it hot, others like it cold. But in general, they all need the same techniques applied to achieve great germination and healthy transplants to get a jump on the season ahead. All need some amount and the correct combination of light, heat, water, and humidity.
Most seed starting resources will tell you to simply fill plastic flats with soil, dump seeds in there and cover with dirt - then wait for life to emerge. Unfortunately, achieving great results is much more technical than that. It took me a while to figure out the best practices to achieve great flower germination rates (90% and above), which is what I’m sharing today.
This is the first part if a three part seed starting series. In the next two parts of the series, I’ll be sharing the pro’s and con’s of starting seeds indoors vs in a greenhouse.
SOIL MIX
Buying a commercial soil mix is best, but be sure to buy a high-quality product that is not treated with synthetic wetting agents. Peat moss should be the main component of the mix - but avoid peat that is too course. Most flower seeds are TINY, and appreciate a very fine texture. Perlite helps with drainage and aeration. For most flowers, you do not need compost-added or amended soil since the seed contains everything it needs to germinate on it’s own. Keep in mind that you can make your own seed starting soil mix, however, I find its much easier and time-saving (and more profitable) to just buy it.
On our farm we usually use pro-mix brand, since it’s affordable and readily available. I’ve heard lots of great things about Vermont Composts “Fort-Vee” mix for seed starting.
When it comes time to use the soil and fill your seedling trays (also known as flats), mix and sift the soil to remove any large debris to ensure textural consistency. Using a shovel in a wheelbarrow, or just sifting with your hands or a feed scoop in a large tote works.
SEEDING FLOWERS IN CELL FLATS
Controversial opinion: I am not a fan of soil-blocking (which is a technique of germinating seeds in “blocks” of compressed soil). I can’t dispute the argued benefits of it (air-pruning of the roots, increase air flow, etc), however, it’s just not economical for a commercial farm. If it works for you, great! But I won’t be discussing that here.
We seed all our flowers into cell trays (aka flats) - these are plastic containers separated into many compartments (“cells”) in which the roots of the seedlings begin to grow. These are known as cut flower “plugs,” since they are transplants grown in these cell trays. It is an effective and efficient technique as the trays are easy to fill, carry, and move from greenhouse to field. They drain well and help prevent fungal diseases, which is a very common ailment to home seed starters.
The cell trays come in many different sizes, and can have anywhere from 24 to 216 cells, which determines how big the cell compartment is. The more cells in a tray, the smaller the cells are.
In general, you use a smaller cell to germinate smaller seeds, and a larger cell to germinate larger seeds. The more you start flowers from seed, the more familiar you will get with the type and size of seed each flower has.
Mostly, we use the 128 trays (each flat has 128 cells in it), but also sometimes use 72-cell or 50-cell trays for flowers with larger seeds (zinnia, cosmo).
FILLING CELL TRAYS
Filling cell trays may sounds like a simple task, however, there is a technique you should be using to prevent issues such as rootbound plugs (flowers that have outgrown their cell, causing crowded root systems), unfilled plugs, air pockets, etc.
The proper filling technique allows as much air retention in the soil mix as possible, while allowing a full cell for proper root development.
Use a soil mix with moisture in it - as long as every tray has a consistent amount of moisture in it, it should be fine. It shouldn’t be bone dry or too wet. The mix is too wet if you grab a fistful and squeeze and see water dribbling out of your fist.
Fill the evenly flats to the brim, taking care that each sell (especially the edges and corners) are filled uniformly. Failure to do this will result is some cells drying out quicker than others - which can’t be easily remedied.
Then, pack the soil into cells slightly by picking up the entire tray about 6” up off a flat, level surface and drop it. Repeat this again. This ensures the bottom corners of the cells are filled with soil so proper root development and plug formation can occur. Do not pack the soil so much that it is condensed!
Next, clear the excess soil from the top using a brush, flat piece of wood, or your hand to scrape it from the top of the cells. You should be able to see each individual cell. If you leave excess soil on the top of the trays, the roots will follow and will grow into one another’s cells: causing a nightmare at transplant time, when you’ll have to remove and separate each cell. The cells should be about five-sixths full.
We batch fill our seedling trays for efficiency, meaning we fill every tray we will seed for the day at the same time. To save space, we stack the filled trays “kitty-cornered,” so we can just grab a tray from the top of the stack to seed. Be sure not to stack the trays so the cells are inserted into each other - this will cause excess compaction of the soil.
SEEDING FLOWERS
As I mentioned before, each flower type has its own preference for seeding, which could be several other guides. I won’t touch too much on that here, except to say that MOST flowers are very small and benefit from being placed directly on top of the soil medium in each cell tray, and then be LIGHTLY covered with vermiculite. Vermiculite is a mica-like, naturally occurring mineral that looks like small, tan shiny or glossy flakes. It is a natural anti-microbial mineral that aids in holding in moisture, while discouraging fungal diseases. It’s available at most garden centers and stores or produce/farming supply places.
Once seeded, we arrange the flats on heat mats and organize them by cell size to ensure even watering. This means we put all of the 128-cell trays next to each other, all the 72-cell trays next to each other, and all the 50-cell trays next to each other.
SEED STARTING ENVIRONMENT
The seeding room or greenhouse environment should ideally be between 64-73*F and 60-90% humidity. In parts 2 and 3 of this seed starting series, I’ll go over how to achieve this further.
The main objective is to control the growing conditions as perfectly as possible in the growing environment. Besides monitoring temperature and humidity, you should allow as much airflow as possible to reduce risk of fungal diseases. Utilize plenty of fans to increase air flow, but note that the fans do not necessarily need to be pointed directly at your seedlings. That could cause them to dry out excessively quickly, so just make sure the air around your seed starting area has plenty of movement. Stagnant air is NOT your friend!
HEAT MATS & HUMIDITY DOMES
Plants require the right amount of light, heat, and humidity to germinate properly. Most plants require a higher temperature to germinate than they do to grow. Each flower has a specific temperature it likes for optimal germination, but for most flowers the temperature is around 65-75*F. I HIGHLY recommend using a heat mat that you will place your cell trays on top of. Heat mats can be set to precisely the right temperature, and I’ve noticed a massive improvement in germination rates when using them. Large, commercial seed-starting mats that can simply be plugged into a 110-V outlet are the most economical if you are starting more than just a few trays at a time, and can be purchased online or through produce or farming supply stores.
Humidity is a major factor that affects germination. Low humidity will result in poor germination, even if you have the right temperature and amount of light. Humidity domes are clear, plastic boxes that are placed over the top of cell trays that trap evaporating soil moisture, causing an increase in overall humidity. I like to use humidity domes for many of our flowers, HOWEVER, you must take caution when using them! Be sure to remove the humidity domes when about 80% of the cells have germinated, or right when they have all germinated, to increase air flow around the seedlings. If you keep the humidity domes on, you will likely get TOO much humidity that will cause fungal diseases to form, and your seedlings will fail. Also always use clean, disinfected humidity domes (and cell trays!).
WATERING FLOWER SEEDLINGS
Water management of seedlings is more of an art than a science, and quite honestly, is much more complicated than it sounds. Watering too little can cause seedlings to dry and die, but watering too much can stunt growth and cause fungal diseases. Properly watering flowers takes much attention to detail. Frequently check your seedlings (we check 3x a day), and know that environmental conditions will greatly affect watering schedules in a greenhouse. For example, sunny days will require heavy watering and sometimes several waterings throughout the day, and cloudy days will require less. Managing all of these factors requires special attention to detail that honestly can only be aquired through experience.
When first watering flowers (before germination or just after germination), use a mister nozzle (I use the “foggit-nozzle brand”) to ensure the seeds are misted gently, and not washed away. The soil mix should stay moist at ALL TIMES. If it dries out, for even a short amount of time, your germination rate will suffer.
Once germinated and a little more established, I love my “wonder waterer” wand, and I highly recommend you get one for watering your seedlings. This wand “breaks” the water droplets up so they are fine and gentle, and do not wash away seeds. Water the trays in a long, sweeping motion to ensure uniform coverage, so every cell gets the same amount of coverage. Watering will need to happen TWICE: once to moisten the soil, and another time to water deep down. You’ll notice different areas of your seed starting environment will dry out at different rates. For example, trays on the edges of greenhouses or closer to the heat source generally dry out faster.
Remember how I said we organize all our trays on the heat mats according to cell size? This is because they will dry out at different rates depending on cell size. Larger cell trays (such as 72-and 50-cell trays) will retain more moisture, and therefore dry out more slowly than smaller cell trays (128’s), which have a tendency to dry out very quickly. These different sized trays will be watered differently, so it’s easier and more efficient to keep them organized.
Finally, be extra cautious not to over-water. Overwatering can lead to fungal diseases, especially “damping-off” when soil stays soggy for too long, and it can be devastating. For this reason, I am not a fan of “bottom-watering,” which is a process where you take an entire tray of seedlings and place it in a larger tray that has about an inch or so of water in it. The soil cells “wick” up the water from the bottom of the large tray. In my experience, it’s very difficult to achieve optimal moisture levels with this method, and it frequently leads to development of fungal diseases and poor flower germination. That being said, I know other growers who successfully bottom-water, so it can definitely be done properly, as long as you pay very close attention and carefully do not over-water.
TIMING
Seedlings are ready to be hardened-off (slowly adjusted to outdoor conditions) and transplanted outside when their roots fill their cells fully, so the roots allow the soil “plug” to hold together well and be fully-formed when removed from the cell. Transplants (aka plugs) should not root-bound. To know if they are at the correct stage, gently tug at the base of one of the seedlings in a cell. The plug should come away cleanly from the cell, and hold its shape and form. If it doesn’t and soil falls everywhere- they need more time to establish a root system. If the plug gives you a lot of resistance when you tug, and comes out showing excessive, crowded roots, then you’ve kept them in there for too long.
At first, it can be difficult to get the timing right. I have a chart I have formed over the years that I use on my farm as a reference for how long seedlings should be held in the greenhouse until they are ready to be transplanted outside. I use this for my crop planning sessions every season. There are some general guidelines out there (a guide for another day), but I’ve found that the amount of time a seedling should remain in a cell tray greatly depends on the environment it’s grown in, so it’s a good idea to keep good notes.
INDOOR VS GREENHOUSE SEED-STARTING SET UP
You can start seeds in a variety of different environments, but the most popular are indoors under artificical lights or in a greenhouse structure (a.k.a. nursery).
In part 2 of this series, you’ll learn about starting flower seeds indoors, under lights, and in part 3 you’ll learn about essentials for setting up a greenhouse system for starting flower seeds. The pro’s and con’s of an indoor-vs-greenhouse seed starting set up will also be discussed.
I’d love to know if you have any specific questions regarding starting flower seeds indoors vs in a greenhouse, or anything else related to seed starting. Drop your questions below and I’ll do my best to answer them in the upcoming guides.
