GENERAL FACTS ABOUT GROWING RHUBARB
This page is an comprehensive introduction to growing rhubarb. It is intended for both farmers and gardeners interested in a relatively deep introduction to rhubarb. If you are seeking straightforward planting instructions, visit Instructions for Planting Rhubarb.
This information is offered in the hopes of affording opportunity for us all to be better rhubarb growers. It will hopefully serve as a repository of general rhubarb knowledge. I welcome any comments, corrections, or contributions you can offer and will amend this document and give credit as appropriate. It would be of value if you can share experience or knowledge different from my own or introduce relevant topics not found here.
The knowledge herein comes from personal research and experience growing 20 different varieties of rhubarb over a 20-year period, commercially for seventeen. Some of the most creative insights, however, were generously shared with me by others and are noted as such. I hope you find this useful.
Rhubarb plants vary greatly in several characteristics. These include stalk size and color, Leaf size and leaf vein color, annual yield, vertical tendency, plant longevity, speed of growth, flower (bolting) frequency and type of flower stalks, root size, skin thickness and texture, fiber rigidity, susceptibility to disease, acid concentration and general flavor. The following bullets enumerate general rhubarb facts and address the above characteristics in the context of those topics.
1) Rhubarb is a perennial vegetable, suited to colder climates such as USDA zones 5 and colder. There has been some success developing varieties better suited to warmer climates. They are generally faster growing and shorter-lived than traditional, cold-weather varieties. They also have lower yields. Wild rhubarb has been found in a desert…no stalks, just leaves lying on the ground to direct moisture to the central root. Noteworthy is the fact that rhubarb is often referred to as a fruit. This is because of its most common use, which is in desserts. It is high in fiber, calcium, and vitamin C.
2) Noteworthy is the fact that some characteristics change for individual plants as they age from year to year and even during a growing season. Some characteristics are more influenced by weather than others. I have seen pure red plants remain red for 10 years, only to have a few partially green stalks emerge in the 11th Mild plants have years when they are suddenly very acidic then return to mild the following year.
3) Rhubarb varieties are grouped into red and green varieties. Red is more common in US grocery stores. Apparently, this is because:
A) Red varieties give the appearance of freshness longer. They have thicker skins and tougher fibers, thereby remaining firm longer at room temperature. Green varieties tend to
dehydrate faster, going limp and appearing less fresh sooner. I’m unaware of reasons to believe reds spoil more slowly than green. Rhubarb can be rehydrated like celery, by immersing it in water. Not sure of the benefit as few people eat rhubarb raw, without sugar, like celery. Green rhubarb that is refrigerated soon after picking will remain firm a week or so.
B) Green stalks can be mistaken for unripe red. In fact, red and green stalks have their characteristic color at all stages of growth…red stalks start growth as red. Same for green. Young green plants, however, can produce young red stalks that turn green as they mature. The telltale difference in young plants is in the leaf veins at the base of the leaf. They will be green in green varieties, tending redder in true red varieties. Most rhubarb is green inside regardless of external color.
C) Culinary applications sometimes prefer red color. While the inclusion of strawberries in rhubarb recipes was not motivated by color, it might well explain it for modern cooks. (The
marriage of rhubarb and strawberries started over a century ago when a British strawberry farmer made the first strawberry/rhubarb tart in an effort to stimulate more strawberry consumption. Prior to that the stalks were not consumed. Until then, rhubarb was grown for the roots, a laxative.)
4) Ripeness is not a matter of stalk color or size. The maturity of the leaf is a better indication of the ripeness of the stalk. Look for leaves that are fully open and beginning to show signs of age…e.g. relaxing or with some bug or critter bites. Contrast with new ly opened leaves is often obvious. There are normally enough leaves at different phases of growth to tell the difference.
5) There are an unknowable number of rhubarb varieties, hundreds at least. Rhubarb hybridizes easily and with great variation. Most commercially available plants, from nurseries and catalogues, are started from seeds.
6) Rhubarb plants appear to reproduce both from seeds and from shoots off spreading roots, though the later is uncommon. Those coming from roots should be true-to-type, as opposed to the wide variation seen from seeds. They can also be manually propagated by digging and splitting a larger root mass to produce two or more identical plants. That is the most reliable way to create true-to-type plants. Best not to split plants younger than four years old. Cloning is becoming more common, but the robustness of such plants has yet to be proven and published. Rhubarb can be split and/or planted any time during the growing season. However, the best success is achieved by doing so early in the year, just befor
e new growth appears and after the soil is workable.
Propagation by seeds is sexual reproduction…children are not identical to the parents. The true ancestry of most plants cannot be known with any certainty. The principle of regression toward the mean is important to understanding seed-propagated rhubarb. For example, seeds for a phenomenally productive, robust plants will likely produce plants across the full spectrum. Most, however, will be average with a few winners and many completely undesirable ones. Even if one characteristic is pass on, others may pass on. Also, seed-propagated plants often don’t show their true character until 1-2 years old. Out of 100 Victoria seedlings, we selected the best fifteen after one year. Two years later, the best of the fifteen revealed itself to be a dud. Instead of producing 90 lbs. of delicious, large stalks, it produced 15 lbs. of small, but delicious, stalks. Of the fifteen, 4 became large like the parents but began to fade after just 7 years. The parents are still strong after 17 years.
7) There are no enforced standards for rhubarb varietal names. Some are named in a sincere effort to associate them with similar traditional varieties, such as Victoria, Turkish, Crimson Red, etc. Others are named simply for market appeal and may have no resemblance to the variety for which they are named. Some people make up new names to reflect the fact their variety does not resemble traditional varieties or for marketing reasons.
8) Rhubarb leaves are toxic to humans. However, it would take 5 kg to kill an adult. Similarly, most wildlife find then distasteful. The leaves contain an excess of oxalic acid. Virtually all vegetables contain oxalic acid but in lower concentrations. The stalks are the food. Roots were traditionally dehydrated and a common ingredient in medicines in the era when illnesses were more mysterious and purging the body was common in many medical remedies. Powdered rhubarb root is still on the market. One cannot help but wonder if it is made from appropriate roots as only some varieties were effective as laxatives. Anecdotally, we have some seed-propagated plants that the deer love. They eat only the leaves, ignoring the stalks and the much larger plants nearby. Notably, all nearby plants were from the same batch of seeds. Another example of the variability of seed-produced plants. Fortunately, none of the deer-candy plants are worth much as producers of human food.
9) Rhubarb is best harvested by pulling the stalks rather than cutting them. Slide your hand down the stalk toward the base. Grip the stalk just tightly enough so your hand does not slip…tighter can result in breaking the stalk where you grip. Lean back and gently move the stalk side to side. Some will still break. Pulling yields more food and gives the plant a chance to recover and grow another stalk…should you be in a climate where a second harvest is viable. A note of caution: Some varieties have such small or narrow roots that pulling the stalk also pulls up the root. Those stalks should be cut.
When starting new plants, it is ill advised to harvest before the plants are in situ at least 2 years old. Harvesting too soon will stunt the growth, possibly even kill the plant. It needs time to establish a root mass sufficient to recover from picking. As a rule of thumb, harvest no more than half the biomass annually until the yield is either what you want or maximized, whichever comes first. But never start harvesting before the plant has been in the ground two years. Three is better. Unless, of course, your plan is to replant frequently and harvest to death.
10) For harvesting roots in quantity, one efficient technique involves growing and harvesting them like potatoes1. Create raised rows of amended soil the width of a potato harvester and
about a foot high. Scatter seeds and broadcast water. Grow for 1-2 years, culling the less desirable plants. Harvest in late-winter/early-spring, before new growth begins, as you would harvest potatoes. A snap with a potato harvester. Poof, scads of rhubarb roots to wash/treat as you like. Probably not worth the investment on a small scale.
11) Bolting (producing flower stalks) is one of the most variable features of rhubarb. Flower stalks are easily distinguished from food stalks. They are cylindrical and hollow. They often also have one or more food stalks branching off them, some at the base but more frequently further up the stalk. General wisdom is to cut and remove the entire flower stalk as soon as they appear. Do not pull as they are much more firmly attached to the root. This is to allow more energy for food production. Of course, let them grow if seed production is the goal. A small, crescent-shaped blade is best as it helps avoid nicking adjacent food stalks. These cylindrical stalks serve two purposes. They allow the plant to project the flower above the leaves for better pollination. They also store water. If you can control the water available to the plant, the later purpose is mute. An indication of the large mount of energy taken to produce these flower stalks: I have witnessed Victoria flower stalks, 1.5” in diameter, grow 4” in one day. Images of Day of the Triffids…man-eating plants from outer space.
The size and number of flower stalks is highly variable. The worst are the plants that spend energy growing scores of small flower heads that do not push up above the leaves (e.g. Turkish). Indeed, those sometimes even have multiple flower heads on a stalk, some coming out at the base. A real time-sink if you are trying to produce seeds from another variety nearby. The best are varieties that produce only a few, large flower stalks and they all protrude high above the leaves (e.g. Victoria). Victoria flower stalks are easily spotted and removed or harvested for seeds. Better still are varieties that only produce flower stalks once a decade or two (e.g. Colorado Red). The food stalks originating off the flower stalks tend to be more acidic than the normal food stalks but are equally edible. However, if you cut the flower stalk above the food stalk some varieties produce another flower head from the junction. Indeed, some varieties produce another flower head from the base of a food stalk even if you cut it at the base. Most annoying.
12) For seed production, the biggest challenges come from the fact that the seeds will not mature simultaneously, and birds will greatly enjoy the feast. If you wait for the majority to mature, then many will already have fallen to the ground and birds will begin to show interest in the ones still on the plant. A proven technique is to envelop the maturing seeds, once fully pollinated, in a sack made from bridal veil material2, gently tied at the base around the stalk. Then you can wait for all seeds to mature before removing the stalk. The sack will catch any seeds that fall. This greatly increase both the quantity of seeds harvested and the percent of viable seeds. Also reduces, if not eliminates, the number of unwanted seeds that may germinate on the ground in years to come. Another challenge, addressed by the sack, is the difficulty in cutting and removing the seed stalk w/o jarring it. Jarring a mature seed head will send seeds flying.
13) Vertical tendency is the tendency for all stalks to grow vertically, verses along the ground. It is mostly of interest to farmers as plants with a strong vertical tendency are more efficiently harvested. Indeed, a vertical plant is required for known mechanical harvesting techniques. However, plants that exhibit an entirely vertical character tend toward lower yields than some varieties that exhibit both tendencies. Yield is of more interest to gardeners.
14) Rhubarb diseases and pests are few and uncommon. They can suffer from a fungus in more humid climates that causes the base of the stalks to weaken. Also in more humid climates, browny mildew can produce small dead patches on the leaves. The can be a problem, particularly in areas where doc plants are numerous (aka Dock plants). Best to destroy any doc plants around your rhubarb. Docks are hardier than rhubarb and provide a breeding ground for the beetle. I have sprayed some Docks with Roundup only to have them come back every year. I think they like the stuff. (Best to keep Roundup away from your rhubarb plants.) There is an odd tendency for some rhubarb leaves to turn red, either entirely or in part. It does not appear to harm the plant or spread to other plants, but the stalk of an entirely red leaf will die. I have sent leaves for analysis to the extension service with no definitive results. However, it can kill a young seedling. Best to remove such leaves and stalks from seedlings.
Leaf eating insects are not likely a threat. I have seen caterpillars attack rhubarb plants, only to witness their bodies dissolve from the excessive acid. We have had plagues of grasshoppers but little evidence of damage to the rhubarb leaves, possibly for the same reason. But I grow in a semi-arid environment. More problems likely occur in humid climates.
15) Rhubarb is a robust and adaptable It can grow in full shade, full sun, bad and good soil. However, for optimal yield and quality, it requires lots of sun, not too much water, and rich, deep, well drained soil. A plant that can yield 90 lbs. of rhubarb in proper conditions may yield only a few celery-like stalks in poor conditions.
16) Rhubarb is a heavy feeder and the roots can go 18” deep. It is best to ensure your soil is rich in nutrients and organic matter before planting and then replenishing the nutrients annually. Fertilize twice annually, at the start of the growing season and after the first harvest. Like all growing, the best fertilization regime is based on soil tests. In general, you’ll want high nitrogen and a balance of potassium and potash, such as 15-5-5. For my organic farm I replenish the potash and potassium with compost. But I must supplement the nitrogen with a 12-0-0 product, amino acids extracted from soy. The optimal growing space also depends on variety. Our larger plants occupy an area eight in diameter and grow four feet tall. Smaller ones require only a two-foot diameter. Prepare the soil accordingly. Remember, amended soil will compact, often compressing to two thirds the initial, loosely amended depth.
17) Optimal sunlight is tied to ambient temperatures. Plants in full sun will not enjoy high ambient temperatures. They will wilt mid-day. Wilting is natural protection from overheating and not harmful to the plant. However, they do not grow while wilted. Also, the heat can impart an off flavor to rhubarb. Creative planning might help. For example, shade cloth in the summer or situating the plants where they were shaded from the mid-day summer sun while in full sun in the spring could improve the situation.
18) Avoid over-watering One of the few ways to kill it. New transplants require more frequent water than mature plants. New plants have fewer and more shallow roots. For mature plants, the roots should partially dry between waterings to allow soil aeration between waterings. In our semi-arid climate, it is time to water if the soil is dry an inch or so below the surface. Best not to plant rhubarb near lawns and other areas that require weekly watering. On our farm, we deep water the plants 3-6 times annually. Of course, they require more frequent watering when first planted as the root system will have been damaged in the planting process. I know someone who could not kill their rhubarb plant with repeated applications of Roundup (probably too dilute). But drowning it worked.
19) Soil high in clay is anathema to rhubarb. Clay soil has the same effect as over-watering. The plants will begin to grow for a year or two and then begin to diminish. Clay holds too much moisture and allows insufficient aeration of the soil. The optimal soil will be high in organic material, drain well, and rich to a depth of 2 feet. That assumes you intend the plant to remain in place for its life. Some farmers plant fast-growing varieties annually and harvest them to death with the first harvest. Obviously, such a scheme has different requirements for soil.
20) Plant longevity is difficult to predict. In general, plants that tend to grow faster also have fewer years of good yield. Younger plants tend to begin growth sooner each year and remain growing later into the growing season than older plants. Large, mature plants tend to fade slowly. They do not have a large harvest one year and die the next. Fast growing plants, however, do fade quickly.
21) Flavor also varies by variety. We have one variety that has a sandpaper skin, very little acid, and a citrus aftertaste. Almost like eating a mildly tart apple. Another has no detectable acid and is almost sweet…stalks are the size of pencils and a yield of one pound, you will never see it in stores. Most rhubarb is too tart (acidic) to enjoy raw and the underlying rhubarb flavor is masked by
the tartness. The natural flavor of rhubarb, if you can get past the tartness, is quite nice. Most people, however, have not tasted that flavor. The tartness motivates over sweetening of many rhubarb dishes and people who like rhubarb dishes sometimes have only tasted the sugar, which most of us like. Try making rhubarb tea and sweetening incrementally until you suppress the tartness w/o masking the rhubarb flavor. Alcohol drinks can also exhibit the true flavor or rhubarb. In a margarita, for example, the triple sec or Cointreau can cancel the tartness without destroying the rhubarb flavor.
Do you have knowledge or experience you would care to add? Points you’d like to expand? Please contact me.
1: Using a potato harvester for mass production of rhubarb roots was suggested by Colin Clayton, of French Harvest, Australia
2: The idea of a wedding-veil sack for harvesting rhubarb seeds was conceived by Donna Duncan, of High Altitude Rhubarb, Colorado USA