Rhubarb is commonly started from root divisions or by planting rhizomes, but it also can be grown from seed. You’ll need to be patient if you’ve chosen seed propagation, which can take up to two years from planting to harvest. Rhubarb thrives and produces best in cool regions of the United States.
Prepare a well-drained planting site in full sun about a month before starting your rhubarb seeds. The seedlings can be moved outdoors when the weather warms to sustained temperatures of about 45 degrees F during the day and 35 degrees F at night. Spread a 2- to 3-inch layer of well-rotted composted manure over the site and work it into the top 10 inches of soil. Keep the area free of weeds.
Test the soil and adjust it to the ideal pH range of 6.0 to 6.8. Rhubarb will grow in acidic soils as low as 5.0 pH, but it will produce better in a more alkaline environment.
Soak the seeds in water for a few hours. Pack a peat pot firmly with seed-starting mix. Water thoroughly to saturate the medium and the peat pot. Push down the medium with your fingers to squeeze out excess moisture.
Plant two seeds about 1 inch apart in each peat pot. Use tweezers to push them about three times their length into the soil. Use a plastic spray bottle to spritz the medium with water so that it’s evenly moist but not saturated.
Seal each peat pot in a clear plastic bag and poke a few holes in it to provide circulation. Set the pot on a warm, sunny windowsill and keep the planting medium evenly moist. Your seeds will germinate faster if the room temperature is above 70 degrees F, in four to seven days.
Remove the plastic bag when the seedlings are 1 inch tall. Pinch off the smaller of the two at soil level when they‘re 2 to 3 inches tall.
Plant the seedlings at least 3 feet apart in your prepared site, at the same soil depth that they occupied in the peat pots. Water gently to avoid disturbing them, and keep the soil evenly moist.
Cover each seedling with a paper cup during the hot afternoon hours each day for one to two weeks. This will acclimate the seedlings to the outdoors without being hardened off first.