From a first time gardener to seasoned flower farmer, growing dahlias from seed is a rewarding experience. There is a surprise in every seed that grows a plant producing its own unique variety. For the sake of simplicity, I will post two different types of germinating methods.
Option 1: Soil Starts
Plant seeds 1/4'' deep into damp potting soil. Seeds like a warm bottom, so a 75 degree Fahrenheit thermostat controlled heat mat is a good idea and will set you up for success. Use a thermometer (digital kitchen thermometers work well) to monitor your soil temperature if you do not have a thermostat. You don't want to cook your seed! Germination occurs between 2-20 days though most will germinate between day 5-10. I place a dome over the tray with the top vents open until day 14 or so when most seeds have germinated. Leaving the humidity dome on for longer just encourages mold and mildew growth. If needed, move your seedling tray within 4" of your lights to get them started without stretching. 12 hrs of light work well. Germination rates vary for dahlia seeds so you may end up with open cells.
Option 2: Paper Towel Method (My Preferred Method)
I like this method because my indoor seed germination space is limited to a 5 tiered shelving unit that lives upstairs in my dining room during late winter/early spring. I don't have the luxury to have unused cells in my seedling trays. Germinating in damp paper towels allows me to move all viable seed into the seed starting trays (I use 50ct plug trays with a 1020 tray/ no holes to contain all moisture) and not waste space. This does require daily attention while the soil start method is a "set it and forget it" operation.
Clean your hands and work station before handling the towels and seeds. Any bacteria that gets mistakenly transferred will be living under perfect conditions to grow and infest your seed. Fold blue paper shop towels in half. You may cut the towels in half to be easier to handle and hold a more modest amount of seed. Make sure your towel will fit the container you'll be using to keep the seeds while they are germinating (zip plastic bag, dish sealed with plastic wrap, or a lidded container). Dampen the paper towel with a spray bottle. You want the towel damp, but not dripping with water. I find using the facet to wet the towels provides too much moisture. I use slips of waxed paper labeled with a Garden Marker to identify my individual paper towels if I am keeping track of varieties. Depending on the marker, if you write on the towel itself the ink can bleed and become illegible.
Once your towels are dampened and labels made, its time to add the seed. Be conscience to only add seed and not chaff or other organic material. Arrange your seeds on the towel so the seeds aren't touching. Fold the top half of the dampened towel over the seeds and gently press making sure the towel has contact with the seeds. This helps keep the seeds from falling out as you take the towel in and out of your preferred container. I have stacked 12 towels at a time will no ill effect. Place all your seeded towels in your container and seal shut. Place in a warm area. I either place on a 70-75 degree heat mat or on my shelving unit. My shelving unit lights will gently heat the shelf a few degrees above the ambient temperature when on. The 12ct bundle is a nice size to handle as you do your daily germination checks.
(Photo illustrates the first 24hrs of root growth)
On day 2, begin checking your towels for germinated seed. The blue towels work great because 1) the color provides contrast to allow you to quickly see the tiny white roots and 2) the towels are thicker, thus retaining moisture better, than the (cheap) paper towels normally in my own household. I have 50ct plug trays pre-filled with damp seed starting potting soil. I use the end of a chop stick to make a small 1/4' indent into the center of each plug. Place a humidity dome with open top vents over the plug tray.
I like using simple white tree tags to individually label my dahlia seedling. I write the germination date, its own unique number so I can keep track of it all season, and the parentage. Place the solid end in the soil. If the punched end is placed in the soil, you will have roots intertwined into it causing unnecessary frustration at planting.
During my daily germination checks (my most favorite time alone, with coffee, before my family wakes up!), I gently pick up each seed with popped a white root and place root down into the chopstick made indents. The seed can stick up above the soil line slightly. Germinated dahlia roots will quickly form tiny hairs that will try to burrow into your towel within a day. If your germinated seed is stuck, its better to gently cut the towel to keep the root intact to plant. Broken roots equals a dead plant. If the seedlings are left unattended, by day three post germination you'll have inches long roots and seedlings trying to leaf out in your towels. Enjoy the moment of discovery and check daily. Germination can happen as soon as day 2 and as late as day 20. The majority will germinate between day 5 and 10.
I fill up 50ct plug trays at a time. Once a tray is filled and the seedlings have all emerged with their first set of leaves, I remove the humidity dome. I have my lights on 12 hrs a day. My home's ambient temperature is 68-72 degree Fahrenheit.
I start my seeds 4-6 weeks before my last frost date, simply because I don't have a greenhouse (thus the space) to bump up my seedlings to bigger pots before planting. This minimized a lot of stress for me to not worry about leggy, root bound plants. After hardening off in mid to late April, these seedlings are directly planted in the field from the 50ct plug trays.
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