Starting tiarella from seeds will require you to prepare a sprouter, or terrarium. You may be able to start them in pots, but the controlled humidity of a sprouter will yield much more satisfactory results. As long as you have a few ordinary tools and household items, the steps outlined below will show you how to successfully germinate tiarella seeds.
Step 1: Prepare Plant Cups
Foam flowers, or tiarella cordifolia, need moist, rich soil to sprout from seeds. Remove the top of the egg carton, and cut a section of the bottom cupped portion that will fit inside the soda bottle. Use potting soil mixed with sifted compost and fill each of the egg cups 3/4 full. Do not pack the soil down.
Step 2: Prepare the Sprouter
A terrarium for seed germination is nothing more than a 2 liter bottle with a vertical cut. Lay the bottle horizontally and cut a slit all the way down the side of the bottle. Make one cut on either end of this slit that runs 1/3 the circumference of the bottle, forming a flap that can be folded open and out of the way. Add enough potting soil for the egg carton to sit level on the bottom and dampen the soil well.
Step 3: Starting the Seeds
Use 2 or 3 seeds per egg cup. Drop them on the moist soil, and cover them with a thing sprinkling of sifted compost or potting soil. Spray lightly with water. Add 1 ounce of water to the soda bottle, and insert the seed cups into the bottle. All that is left to do is to seal the bottle and place it in a safe place.
Step 4: Seal the Terrarium
Use duct tape to seal the flap closed. Be careful to get a complete seal without folds or wrinkles that allow air to get in or out. The way a terrarium works is to absorb heat from the sun, which causes evaporation in the soil. This in turn creates a mini ecosystem that is high in humidity but free from outside interference. If it is properly sealed, a sprouter or terrarium can go for many days without taking any action at all. Starting tiarella can take up to 30 days or more and so this setup will make things easier for you.
Step 5: Allow Seedling Development
Cut the duct tape along the slits you have made in the bottle. This will let you use the same bottle multiple times. You can cut off the end and remove the seed tray if you want.
The plants can remain in these cups until you are ready to place them in the garden or flower planters. A good time frame for final planting is about 2 weeks after the seeds have sprouted. Loosen the soil where they will be planted and insert the original soil with the seedling.