Forcing rhubarb allows gardeners to harvest sweeter-tasting stems much earlier in the season, sometimes as early as late winter. While this practice is popular in England, few American gardeners employ this technique. Yet, all it takes is a way to exclude sunlight from the rhubarb crown.
How to Force Rhubarb
There are several ways to force rhubarb, but they all rely on three key factors:
- Chill hours – Rhubarb requires exposure to chill hours, or cold temperatures below 37 degrees F. (3 C.) for a period of 7 to 9 weeks, or 500 hours below 49 degrees F. (9.4 C.).
- Warmth – The return of warmer temperatures triggers the rhubarb to break winter dormancy and begin growing.
- Complete Darkness – Lack of light forces the rhubarb stems to grow taller. The plant puts more energy into the stalks, thus the leaves are smaller and the stems taste sweeter.
Follow these steps to harvest rhubarb during the winter months:
- At the end of the growing season when the plants have died back naturally, cut away the old leaves and dig up a portion of each crown. You can also use the entire crown but leave some crowns in the ground for forcing in future years.
- Place the crowns in the refrigerator to meet the chill hour requirements.
- Once the chill hours are met, plant the rhizomes in pots and cover with soil, manure-compost and straw.
- Place the pots in a totally dark, cool location with an ideal ambient temperature around 50 degrees F. (10 C.). Cover the pots, if necessary, to ensure the crowns receive no light.
- Keep the soil moist, but not soggy. The forced rhubarb can be ready for harvest in as little as 4 to 6 weeks.
This is similar to the indoor method except the crowns are potted and placed in an outdoor shed or garage after being exposed to cold temperatures. The crowns can also be potted and placed in an outdoor shed while their chill hour requirements are being met.
Either way, the pots can be moved to a slightly warmer (50 degrees F.) location, or a heat source can be added to the shed or garage to trigger growth. Alternatively, the pots can remain in place until rising outdoor temperatures in late winter or early spring stimulate the crowns to grow. With these methods, it can take 7 to 8 weeks for stems to reach a harvestable size.
What is a Rhubarb Forcer?
Rhubarb can also be forced in the garden. This method doesn’t require the crown to be lifted, potted or replanted. But it does require a rhubarb forcing jar to completely block all light from reaching the crown.
A forcing jar is a stylish bell-shaped pot with a removable lid. Traditionally made from terra cotta, plastic forcing jars are also available. The latter has the benefits of being lighter weight, non-breakable and easier to store. Although not as decorative, any light-retarding 20 inch (50 cm.) tall container, such as a bucket or trash barrel, can be used.
The rhubarb forcer can be placed over the rhubarb once the crowns have been exposed to a few cold snaps. To prevent root rot, be sure all old vegetation is cleaned up and make sure the ground isn’t saturated before covering the crowns with the forcing jar.
Any holes or cracks in the forcer should be taped over and the jar can be wrapped in old carpet, bubble wrap or filled with dry straw to trap heat. Once the rhubarb starts growing, the jar lid can be taken off during the day to encourage longer stem growth.
Additional Rhubarb-Forcing Tips
- Harvest forced rhubarb stalks when they reach a length of12 to 18 inches (30-46 cm.).
- Expect yields from forced crowns to be about half what the same sized crown would produce if grown naturally.
- Replant forced crowns, but allow them to grow naturally for at least a year before forcing them again. It’s best to rotate through your rhubarb plants and choose different crowns for forcing each year.