Beginner’s Guide to Seed Starting Cut Flowers Indoors

Want to start seeds but not sure where to start? I’ll show you all of my tips and tricks in this post, to get you growing in no time! Since I am not running any workshops at this point, I thought a blog post to cover what I usually teach was in order.

A lot of what you can grow indoors depends on the following factors:

• How much space you have
• The amount of time it takes before the seedling can be transplanted
• Consistent watering schedule 
• Ambient air temperature
• The type of growing medium you’re using etc… 

Be sure to comment with any questions you have, I’ll try to answer as many questions as possible, but I want to make sure you are able to have success growing your own cut flowers.

which seeds should you start indoors?

Here are a few of my favorite cut flower varieties for starting indoors:

• Benary’s Giant Zinnias
• Cardinal Basil
• African Marigolds
• Cramer’s Celosia
• Pampas Plume Celosia
• Double Click Cosmos
• Pro Cut Sunflowers

These are really easy as far as seed varieties go to start indoors yourself. I have more lists with trickier varieties that you can see by clicking HERE. But if you are just getting started, I’d highly recommend trying these particular flowers. These varieties also do well direct seeded out in the garden, so it’s not as critical to succeed with them if you end up struggling.

where to buy cut flower seeds?

Personally, I am a big fan heirloom seed companies, like Baker Creek, Seed Saver’s Exchange, or Sand Hill Preservation Center, but I do know that they don’t always have the selection of cut flowers that you may need. Johnny’s Seeds is also great, and if you have your Business Tax ID number, Geo Seeds and Ivy Garth are fantastic.

Supplies you’ll need to start seeds indoors:

To successfully start seeds indoors you’ll need a growing rack. You can grow seedlings in windows, but it is very hard to get them enough light, and they will often get leggy and try to reach for more sunlight.

By setting up your own growing rack/shelving unit you’ll be able to guarantee that the seedlings get the conditions they require to grow well.

• Shelving unit
• 4 foot shop lights (2 per shelf)
• Fluorescent lights (one warm bulb and one cool bulb per shop light)
• Heat mats
• Fan
• Outlet timer
• Chains (often provided with the shop lights) and S hooks
• Temperature controlled area (relatively)

Most seeds germinate between 60-70 degrees, and soil is typically 10 degrees cooler than the ambient room temperature. The heat mats help keep the soil warm, and the fan reduces stagnant air which can lead various problems.

Here’s a peak at one of my past set up’s – to give you an idea of how it all comes together.

You can find wire shelving at most hardware stores: Lowes, Home Depot, Sam’s Club, etc… all have similar products. You want one that is 4 feet wide, so you don’t waste any of the lighting. Shop lights and bulbs are available at any home supply store and even WalMart. I bought basic heat mats from Amazon. You can also DIY heat mats with rope lighting.

Using florescent shop lights instead of Grow Lights saves a ton of money, which is probably one of the reasons you’re starting your own seeds anyway!

To set up the rack you’ll want to follow these steps:

1. Build the shelving unit
2. Insert one fluorescent cool bulb and one warm bulb in each shop light
3. Hang two shop lights per shelf, this is pretty easy if you have a metal shelving unit. My husband has even installed small metal hooks on cheap plastic shelves in a pinch when we ran out of metal shelves
4. Lower the lights with chains so they are about 4-6″ above the seedlings
5. Set up a timer to automatically turn off and one the lights, I typically keep them on 16-18 hours per day

How to start Cut flower seeds indoors

Two of my favorite methods of sowing seeds are soil blocking or using potting soil in grower’s trays (or flats). Both have their advantages and disadvantages. Additionally you can use individual pots, cups, or even egg cartons, but for this post I’ll focus on trays and soil blocks since they are the most efficient if you are growing a large quantity of seedlings.

Now you may be like, “hold up, WTH is soil blocking?”

Soil blocking is a process invented by a guy named Eliot Coleman, its the process of using a form to make “block” or cube of soil within which you plant your seeds.

Soil blocking sounds super complicated, and a little overwhelming, but I promise it’s not. However, just because I like it, doesn’t mean it will be the best fit for your gardening situation.

Here’s an excerpt from my book, How to Start a Flower Farm, where I cover the differences between soil blocking and flats,

You will see a lot of smaller farms using soil blocks because they are more space efficient and seedlings can be transplanted sooner. Soil blocks are especially great for small seeds. You can fit 240 little seedlings on a single 12 x 16” tray. The seedlings are typically ready to go out in the field within 4-6 weeks. 

Flats are great because it’s a straightforward process: fill it with dirt. Easy enough. Flats are a standardized size (1020) and come with a certain number of cells. The smaller the cell size the larger the number of cells in that flat. They’re more suited to greenhouse growing but you can try to make it work indoors. Generally you’ll want to start with the smallest cells and as the seedlings get bigger “bump them up” into bigger cells. 

There are two sizes of soil blockers, ¾” and 2”. You can use the mini soil blocker (¾”) for all smaller seeds but for bigger seeds like zinnias, sunflowers, and cosmos you’ll want to either use the 2” blocker or a 128 cell flat. 

There are two sizes of soil blockers, ¾” and 2”. You can use the mini soil blocker (¾”) for all smaller seeds but for bigger seeds like zinnias, sunflowers, and cosmos you’ll want to either use the 2” blocker or a 128 cell flat. 

Supplies you need for soil blocking

• Soil Blocker
• Soil mix (seed starting mix if you are buying pre-made, OR mix 16 cups of peat moss (preferably coconut coir) with 4 cups of compost to make your own)
• Trays (cafeteria trays work well)
• Vermiculite
• Watering can with narrow sprout

To make soil blocks you’ll want to mix your soil with enough water to make it sort of the texture of wet cookie dough. Gross sounding, I know. Basically, you want it wet enough to stick together, but not so wet that it falls apart. I found it challenging to get the right texture at first, but now it’s simply second nature, so don’t be discouraged if it takes a few tries to get it right.

You press the blocker down into the soil and use a tool (like a potato masher) or your hands to help tamp the soil into the blocker. Then you’ll press the hand lever over your tray to release the blocks.

When seeding, you’ll press one seed on top of each block. You can then sprinkle vermiculite lightly over the top of the seeds to help keep the soil moist.

If you end up having a problem with dampening off (when the seedlings spontaneously fall over) you can sprinkle the top of the soil with cinnamon to help prevent it

Water once or twice a day, depending on your ambient humidity levels. Soil blocks should stay moderately damp, but its okay for them to dry out from time to time. You’ll want a watering can or even a bottle with a fairly narrow spout to water around the edges of the tray. This keeps the soil blocks from getting washed away and crumbling.

Supplies you need to sow seeds in flats or pots

• Soil mix (seed starting mix if you are buying pre-made, OR mix 16 cups of peat moss (preferably coconut coir) with 4 cups of compost to make your own)
• Seed starting trays or pots
• Vermiculite
• Watering can

Sowing in flats or pots is pretty simple – you fill them with seed starting mix and plant the seeds. I like to water the soil before planting, I find that this creates fewer problems with absorbency. Tamp the seeds down and either cover with more soil or leave the seed on the surface depending on whether or not they need light to germinate. You can typically find that information on the back of the seed packet. If it doesn’t say, than it that variety of seed may not need anything special to germinate.

One trick to watering your new seedlings is to use a 1020 flat tray without holes under your flat or pots. This will keep any soil from washing away. You can even water from the bottom, pour water into the tray and the individual soil cells will absorb the water.

How long to grow seedlings indoors before transplanting outside

How long to grow seedlings indoors before transplanting out side depends on a couple different of factors:

1. The variety of plant
2. The seed starting method

Some plants only need a two weeks of growing before they need to be transplanted, think sunflowers, or even zucchini.

Others, like zinnias, cosmos, and marigolds will do well with approximately 4 weeks. You’ll need to use your best judgement and ask yourself a few questions: do they have their true leaves? Do they look like they need more room?

The time needed to grow them also depends on how you’ve started them. Soil blocked seedlings are typically ready to be planted out a week or two sooner than those in pots or flats. A good rule of thumb is to plan for 4-6 weeks of growing in soil blocks and 6-8 weeks in flats. Some really tiny seeds need even longer.

You’ll get the hang out it, I promise! The best advice I was ever given about growing seeds was from a co-worker at a greenhouse, he said, “eventually you’ll just know exactly what the plants need. The more you learn, you’ll look at them and be able to see how to take care of them.”

So here’s to a great growing season! If you have any questions or comments please post them below. Can’t wait to hear from you! And if you need more growing help sign up for my complete list of flowers to grow for beginners.