Facts of Rhubarb Growth and Harvest
This page outlines the basic information related to virtually all rhubarb and where/when/how to plant and harvest, general few growing techniques, including forcing indoors, and diseases and insects. See Organic Technology for details of our rather innovative organic techniques. See Phases of Growth for photographs of the plants in various stages of annual growth.
WHERE to PLANT RHUBARB
Rhubarb is a robust and prolific plant in the right climate and soil. Historically it has been a cool-weather perennial. For most varieties a good rule of thumb is that it requires mean summer temperature below about 75o and a mean winter temperatures below 40o. That translates to USDA Zones 1-5. (See USDA Half-Zonesto determine your zone.) The plant needs 2 months of dormancy for a proper rest period during the winter. The cold-weather varieties go dormant in frozen ground. Under those conditions it will normally have no diseases or pests...making it ideal for organic techniques.
In recent years, however, warmer weather varieties have surfaced. Rhubarb can be found in climates such as Monterey, CA. There are commercial varieties, such as Cherry Rhubarb, found in southern California farms1. Such varieties still require a few months dormancy but can be stimulated to provide 2-3 harvests per year. In parts of Australia rhubarb is grown as an annual. Started in greenhouses, planted and harvested to death each year. It is unclear whether the warmer-climate varieties also thrive in cold climates. Our own experimentation with such varieties is in a cold climate. They appear to mature much faster than cold climate varieties. We have yet to see how they endure after multiple winters and are concerned that faster growth rates may translate to shorter lives. But we do not yet have data on persistence and survival rates.
Rhubarb prefers well drained soils unusually rich in decomposed organic material...twice as rich as most other plants would want. For best results the rich soil should be at least 12" deep, 36" in diameter and 30-50% organic material. Earthworms help, too. Rhubarb prefers slightly acidic soils but is tolerant of a wide range of pH and soil conditions, provided it drains well.
Most rhubarb varieties first mature within 2 months of the start of the growing season. The rhubarb season starts as soon as the ground is thawed and the days are consistently above freezing, which is March/April in Black Forest, CO. It can easily endure freezes and snows during the growing season but cannot tolerate temperatures approaching 100oF for very long at all. In high temperatures it will wilt to protect the plant...to keep it cool. Wilting does not damage the plant but no growth takes place while the plant is wilted. In response to prolonged periods of heat, however, the plant will simply give up. It grows best in the higher elevations of Colorado and in northerly states such as Michigan and Washington state...places where most homes require no air conditioner at all. These are generally categorized as agricultural zones 4 or 5. In warmer climates such as Oklahoma, Texas, or the South rhubarb frequently fails to survive even one summer. Unfortunately, the plants need two years good growth to produce viable harvests. Consider that together with their intolerance of warm weather and you can only conclude that rhubarb is altogether unsuitable to outdoor growth in warmer climates...they simply are not a viable annual. However, the plants can be forced indoors with acceptable results. Indeed, this was the preferred way of producing rhubarb during the heyday of its culinary popularity. Forced plants were reputedly milder and better tasting. (See below for more information about forcing rhubarb.)
Methods of commercial production vary greatly. Some farmers, like us, maintain mature plants for years, leaving them in the same soil. However, most such farmers have to replant every 5 years as rhubarb is a very heavy feeder and the soil must be amended. We've found a way, unique to our circumstances, to avoid that. Some of our plants have remained in place for over 12 years. In Australia cloning is common. With cloning new plants are planted every year and harvested completely, to their destruction.
Most private growers, growing rhubarb for their personal consumption and to gift to friends, are best to leave the plants in place as long as they are productive. You might consider having two plants, staggered in age by 3 years, so you'll always have a mature plant from which to harvest. When the harvest from the older plant begins to drop off you can dig it up, amend the soil, and put it right back where you got it. Our experience is that any such activity causes all the stalks and foliage to die almost immediately. However, the plant will soon replace them.
WHEN to PLANT
For the shortest time-to-harvest and the most likelihood of a plant true to the parent you should elect to plant freshly dug root stock from large, good tasting, healthy parents. For best results transplant roots in the spring just as the new shoots are beginning to grow. For the most reliable results ensure that each root has the bud of at least one growing stalk (though this is not absolutely necessary, it increases chances of success). (See phases of growth, "one week after compost blanket removed" for an idea of what the plant should look like when dividing the roots.) Seeds also work but do not often breed true to the parent...you might get better, might get worse. Start seeds indoors two months before the planting season. Start them as you would any large plant...begin in small trays, transplant to pots as they get their first real leaves, etc. Transplant as soon as danger of snow has passed and the ground is thawed. Plant 5 to 10 seeds or freshly started seeds within a circle several inches apart wherever you want one plant and thin any undesirable roots as their nature is revealed. Hopefully, the stronger, larger plants will overwhelm any puny relations and dominate the new growth.
WHEN and WHAT to HARVEST--RIPENESS
Each variety of rhubarb demonstrates ripeness in its own, unique way. For most varieties the color and diameter of the stalks is not an indication of ripeness. Indeed, in some varieties you will find stalks almost pure red outside right along side stalks almost pure green outside and both are equally ripe. Stalk length and firmness is the most reliable indication of ripeness.
Plants reach their optimal quality, in terms both of taste and nutrition, when they are about 2/3 their maximum size. At that size any stalk that appears full length and firm will be ripe. For some varieties, particularly the ones that are red inside and out, the deepness of red in the outside color is usually an indication of the deepness of red inside as well, but not always. These plants are a minority of rhubarb varieties. In our experience the most common variety is also the most flavorful to many palates and is green or Victoria in character. These stalks vary from pure green inside and out...for some of the mildest rhubarbs...to red and green outside, green inside.
It can be difficult to pick the riper and larger stalks on a mature plant. Even at 2/3 full size the leaves are normally so thick as to obscure any clear view of the stalks. As a general rule of thumb I suggest you let the leaf size be your guide to selecting potentially ripe stalks. Start with the largest leaves and pull their attached stalks. If firm and the same length as the majority of longer stalks then they are ripe. If they have begun to hollow out inside...sometimes not visible until you cut off the leaf...then they are too old. The older stalks, not common on plants at 2/3 maximum size, will be more fibrous and usually too acidic with tougher skins.
Harvesting any significant amount before the third year of growth is ill advised if you want to harvest from the same plant indefinitely. Rushing the first harvest will weaken the root system and lead to disappointing future harvests. When you pick a stalk in a young plant the plant will divert energy to replacing that stalk instead of increasing its root mass. Once a large, healthy root mass is established, however, you should still limit your enthusiasm to picking 1/2 of the visible biomass of the plants at each harvest. Weather permitting even organically grown rhubarb can produce two large harvests each year. Non-organic methods sometimes produce 3 harvests per year. In a given year each subsequent harvest will be smaller than the previous and each should be limited to half of the visible biomass. Experience tells us that such a practice will result in increasingly large, strong and healthy plants year after year.
HOW to HARVEST MANAULLY
Encircle your target stalk below the leaf with your hand. Gently slide your hand down the stalk to get as close to the base as possible. Tighten your grip, pull straight out and simultaneously twist and bend about 1/4 turn, 45o, respectively. This technique will reduce the risks of either pulling up the root or breaking off the stalk above the root connection. You want the stalk to break off from where it attaches to the root. This will make room for another stalk to replace it later this season. Harvest no more than 50% of the biomass of each plant in each harvest. Such maximum harvests can be made two, three or four times a year, depending on your goals and growing technology. Such harvests should be 5-8 weeks apart. If you simply can't wait two full years you can often harvest about 1/4 of the biomass during the second year without significant impact on future growth...but this will not be a very large harvest and you're best advised to be patient and wait another year
Rhubarb is ideally suited to organic growing provided the fields are properly prepared BEFORE planting. See the details of our techniques for one suggestion. Growing organically, however, does place smaller limits on the harvests than those of non-organic techniques. We get at most two high quality harvests per year, about 6-8 weeks apart, taking 50% of the biomass each time with the second harvest yielding about half the pounds per plant as the first harvest. Going beyond that would stress the plant beyond the soil's ability to fuel a recovery with continued growth and equally large future harvests.
We also have experience with non-organic approaches. Fertilize the root area and surrounding soil once a month with a hand full of good vegetable fertilizer. Water weekly. You'll be able to harvest about every 4-6 weeks until the plants give up for the season or harvest smaller amounts continuously during the growing season.
FORCING RHUBARB INDOORS2
"Forcing" refers to any process designed to produce a harvest, or blossoms, in conditions ostensibly non-optimal. For rhubarb it is the process of growing rhubarb indoors at temperatures and in soil similar to their ideal outdoor conditions but in considerably less light. The roots are often selected from 2+ year old plants, often grown outdoors. If outdoors, then the roots are often transplanted when the plant goes dormant for the winter. In those conditions, however, the roots must be allowed to rest for at least 5 to 6 weeks before they can be stimulated to resume growth. In colder climates suitable to rhubarb this normally involves digging, splitting and planting root stock in deep pots or raised boxes in late Fall or early Winter, after the plants have been dormant for weeks but before the ground freezes solid. If outdoor conditions don't allow sufficient time for the plants to rest then they can be transplanted indoors earlier. In that case, the room should be kept at or below freezing for 5-6 weeks, to allow the plants to rest indoors before resuming growth.
The roots can be buried under a few more inches of soil than is recommended during normal planting and, to resume growth, the room can be brought to and kept at 55o to 60oF. Never allow the temperatures to approach 70o or you risk exhausting the plant and reducing the quality and quantity of your crop. Forcing is normally done in dark, cool cellars or by inverting empty, large pots over the top of a potted rhubarb if the room is otherwise lighted. The resultant stalks will be lighter in color, more tender and milder in flavor than normally produced by the variety outdoors. Indoor growth rates and yields, however, are considerably less than outdoor production. Greenhouses can be used to force rhubarb. However, the harvest will be similar to those produced outdoors. Rhubarb forced in darker conditions is generally considered of superior quality to outdoor harvests so greenhouse production has always been uncommon. If growing in a cellar or room with windows the windows should be virtually blacked out. Otherwise, the stalks will contort themselves toward the light and be quite unusually shaped and difficult to handle.
Keep the beds moist. Production will fall off sharply in beds that become dry. Harvest is generally within 2 or 3 months of the start of growth. High humidity is also a plus. Decreasing intensity of the stalk color is a sign the roots are becoming exhausted.
Forcing was common commercial practice during the peak of rhubarb culinary popularity.
Forcing in the U.S. essentially ended with World War II but remains in commercial use in Britain. Research into forcing was once conducted at Stockbridge House Experimental Horticultural Station at Cawood, Selby, North Yorkshire, England. Details of their work have proven tough to come by.
DISEASES and INSECTS1
The most serious disease affecting rhubarb is foot rot. It is caused by a fungus which causes the stalks to rot at the base, resulting in the stalks falling over. The most effective treatment is to remove any infected plants entirely and treat the infected ground and any healthy plants nearby with a bordeaux mixture. Bordeaux mixtures are available from many farm and gardening supply houses. Copper appears to be the active agent.
Only one insect is known to cause significant damage to rhubarb, the rhubarb curculio. (Lixus concavus, aka the Rhubarb Weevil) It is a rusty snout beetle about 3/4 inch long. It bores into the stalks, crowns and roots. Controls include: hand picking,; burning infested plants; and destroying any wild doc growing in the vicinity (the beetle also resides in the dock). Burning and removal of the dock is best done in July, after the critters have laid their eggs.
1: "Culinary Rhubarb Production in North America: History and Recent Statistics", Clifford Foust & Dale Marshall, November 1991, HortScience, Vol 26(11)
2: More details are available from the U.S. Department of Agriculture Leaflet No. 354, "Rhubarb Production Outdoors and In", 1954, revised 1958