Rhubarb

Rhubarb (Rheum x hybridum) is one of the most dependable and low-maintenance perennials in the edible garden. Plant a single crown in well-prepared soil, and a well-managed plant will remain productive for 20 years or more — reliably delivering one of the earliest harvests of spring before almost anything else in the garden is ready.

The Pacific Northwest is prime rhubarb country. Washington and Oregon are historically among the top-producing rhubarb states in the US, and the cool, moist maritime climate west of the Cascades is ideally matched to rhubarb's needs: cold winters that satisfy its chill requirement, cool spring temperatures that develop vivid red pigmentation in the stalks, and moderate summer temperatures that extend the growing season without triggering premature decline (OSU Extension EC-797).

The edible portion is the leaf stalk — the petiole — harvested for pies, jams, sauces, and preserves. Rhubarb leaves are toxic to humans and animals due to high concentrations of oxalic acid and soluble salts; they must not be eaten or composted in a pile that will be used on food crops. Discard them — do not eat them, and do not let animals graze on them.

Rhubarb is also a genuinely ornamental plant. Its bold, architectural leaves — sometimes up to two feet across — make it a striking addition to the edible landscape, and the vivid red stalks of red varieties are visually striking from late spring through summer. For a long-lived perennial that takes little space, produces reliably early, and requires minimal attention once established, few vegetables compare.

Rhubarb is cold-hardy and heat-sensitive — the inverse of most garden vegetables. Understanding these requirements before siting your planting is essential to long-term success.

Cold requirement

Rhubarb requires a minimum of 500 hours of winter temperatures between 28°F and 40°F to properly form new leaf buds for the following spring (OSU Extension EC-797). Without adequate winter cold, bud formation is poor and yields decline. Established crowns and rhizomes tolerate frozen or very cold, dry soil for extended periods without damage — the underground portions are substantially more cold-tolerant than the aboveground growth. Growers in Zones 3–8 will have no difficulty meeting this requirement. In Zones 9–10, rhubarb can be grown as a cool-season annual, but will not persist through hot summers or mild winters.

Summer temperature

Summer temperatures averaging less than 75°F support the most vigorous vegetative growth (OSU Extension EC-797). As temperatures consistently exceed 80°F, growth slows significantly; rhubarb will not thrive in sustained heat above 90°F. In the PNW's maritime climate west of the Cascades, this is rarely a problem. In the warmer, drier interior — eastern Washington and eastern Oregon — providing afternoon shade in summer is worthwhile to extend the productive season and maintain stalk quality.

Color and temperature

Cool spring temperatures directly influence stalk color. Low temperatures in spring, combined with variety genetics, promote the development of pink-to-red external and internal pigmentation that most people prefer (OSU Extension EC-797). Warm springs and high summer temperatures cause green petioles even in varieties that are red under cooler conditions — this is normal, not a sign of a wrong variety. PNW growers producing rhubarb in the cool, rainy spring season are well positioned to develop the vivid red color characteristic of high-quality stalks.

OSU Extension EC-797 is specific: a deep, rich, well-drained sandy loam with a soil pH of 6.0 to 6.8 is ideal for rhubarb. If your soil tests below pH 6.0, apply agricultural lime at 5–8 pounds per 100 square feet before planting and work it in thoroughly. If soil tests above 6.8, elemental sulfur can lower pH — contact your local cooperative extension office for region-specific amendment rates.

Because you will not be able to amend the soil deeply once rhubarb is established, thorough preparation before planting is critical. The best approach:

• In well-drained soils, amend a site at least 24 inches across for each individual plant — deep enough to accommodate the roots (OSU Extension EC-797)
• In heavy clay soils, amend the entire rhubarb planting area to prevent moisture retention that could cause crown rot; consider raised beds or mounded planting if drainage is marginal
• Work 2–4 inches of well-rotted compost or aged manure thoroughly into the top 8–10 inches of soil. Fresh manure should be aged at least one year before use (OSU Extension EC-797)
• Rhubarb is a heavy feeder — it produces large, dense leaves and stalks each season and requires a nutrient-rich root zone to sustain that growth

Light, sandy soils produce an earlier crop but dry out faster and require more frequent irrigation and fertilization than heavier soils. Amend sandy soils generously with organic matter to improve moisture retention. Heavy clay soils produce later but generally retain nutrients and moisture well — the priority there is improving drainage to protect the crown.

What to plant: crowns, not seed

Vegetative propagation by crown division is the recommended method for establishing rhubarb (OSU Extension EC-797). Named varieties must be propagated vegetatively — rhubarb is cross-pollinated and seed rarely produces plants that resemble the parent variety. Starting from a named crown division is the only reliable way to get the variety you want. Seeds are used mainly by plant breeders.

Purchase one- to two-year-old crowns, or obtain divisions from an established planting. Each division should contain at least one large vegetative bud with roots attached. Plant divided crowns as soon as possible after division — exposed crowns dry out quickly.

Identifying the eye on a bareroot crown. Bareroot rhubarb crowns arrive dormant and can be disorienting — it is not always obvious which end is up. The eye (bud) appears as a blunt, rounded protrusion on the upper surface of the woody crown, typically reddish to pinkish in color and slightly pointed at the tip. It may be anywhere from the size of a small marble to roughly an inch across depending on crown age. Large vegetative buds are found on the upper portion of the crown (OSU Extension EC-797); the underside is where the fibrous roots emerge. If the crown has multiple buds, they will all be on the same face — that face goes up. Orient the crown so the eye or eyes face upward, and position the top of the crown 1–2 inches below the soil surface. If the eye is already showing a hint of red or pink growth when the crown arrives, plant promptly — the crown is beginning to break dormancy and should not be left exposed or stored.

When to plant

In most climates, plant in early spring as soon as the soil is workable and before significant growth begins — ideally when soil temperature is above 40°F. In the PNW, this is typically mid-March through mid-April. In areas with severe winters, spring planting is preferred over fall because it gives the root system more time to establish before the following winter. Fall planting is possible in mild-winter zones — plant late enough that the crowns won't push new growth before dormancy, but early enough for roots to develop before the ground freezes (OSU Extension EC-797).

Planting depth and spacing

Plant crowns with the top of the crown 1–2 inches below the soil surface (OSU Extension EC-797; Illinois Extension). Place the crown with buds facing upward. Do not bury the crown more than 2 inches — too deep delays emergence and increases rot risk. Firm soil around the newly planted crown to eliminate air pockets, and water thoroughly after planting.

Space individual plants 3 to 4 feet apart within rows, with rows 4–5 feet apart (OSU Extension EC-797; SDSU Extension). Rhubarb grows large — a mature plant spreads 3–3½ feet — and crowding reduces air circulation, increases disease pressure, and limits yield. Remove all weeds from the planting area before planting; rhubarb does not grow well in a weedy environment.

There are over 60 species of rhubarb; most culinary varieties grown in North America areRheum x hybridum, bred for stalk size, color, and flavor (OSU Extension EC-797). Varieties differ primarily in stalk color, stalk size, seed-stalk tendency, and adaptability. Red-stalked varieties are most popular for fresh market and home use; flavor and nutritional value do not vary significantly by color, though red varieties are often perceived as milder and less fibrous than green-stalked types.

Fertilizer

Rhubarb is a heavy feeder that produces large, nutrient-demanding leaves and stalks each season. Nutrition applied one year directly influences the following year's yield and stalk size (OSU Extension EC-797). At planting time, mix a quarter cup of 5-10-10 fertilizer with amended native soil and backfill into the top 10 inches of each planting hole — do not place fertilizer directly against the crown.

In subsequent years, apply a balanced fertilizer (10-10-10, or ¾ to 1 cup of 10-10-10 per plant) in early spring before growth begins, along with 2–3 inches of well-rotted compost spread over the root zone (OSU Extension EC-797). Sprinkle fertilizer around the plant — not directly over the crown — and water in after application. In most established garden soils, nitrogen is the primary nutrient needed; excessive phosphorus should be avoided in soils that already test high (Iowa State Extension). Fall fertilization, if used, should wait until the plant has gone fully dormant to avoid stimulating late-season growth that is vulnerable to early frosts.

Watering

Rhubarb stores moisture in its large root system and needs consistent soil moisture to a depth of 12 inches for productive growth (OSU Extension EC-797). Water deeply — long enough to move moisture several inches down — rather than shallowly and frequently. Allow the soil surface to dry to a depth of approximately 2 inches between waterings. Wilting or limp leaves are a reliable indicator of inadequate water.

In the PNW west of the Cascades, spring rainfall typically covers irrigation needs through June. Supplemental irrigation becomes important in July as rainfall diminishes and temperatures rise — exactly when rhubarb is finishing its harvest window and building root reserves for the following year. Rhubarb grown in cool, rainy areas may need little to no additional irrigation until after the first harvest; areas that are hot and dry will need irrigation most of the growing season (OSU Extension EC-797).

Apply 2–4 inches of clean straw, leaf mulch, or wood chips over the root zone to conserve soil moisture and moderate root zone temperature — particularly important in summer. Keep mulch pulled back from the crown to avoid moisture accumulation that can promote crown rot.

How to harvest

Grasp each stalk near its base and give a slight twist with a sideways pull to remove it without injuring the crown bud at its base. Each bud produces multiple stalks — bud damage directly reduces future yield (OSU Extension EC-797). Ohio State Extension notes that cutting stalks with a knife is debated because the resulting wound can become an entry point for crown rot; the pull-and-twist method is preferable for home garden harvest.

Harvest stalks as they reach usable size before any coarse fiber develops — stalk quality and flavor are highest in early spring when petioles are at their freshest (Illinois Extension). A typical harvest season runs approximately 8 weeks. In the PNW, rhubarb is often harvested in May and again in early to mid-July.

When to stop harvesting

Stop harvesting by late July and do not harvest into August. From late July onward, the plant must be allowed to grow freely — leaves produced in late summer drive photosynthesis that loads carbohydrate reserves into the root system, providing the energy reserve for the following spring's growth (OSU Extension EC-797). Late summer harvest reduces yields and plant vigor the following year. Never harvest more than one-half of the plant's stalks at any one time, to ensure the remaining foliage can sustain the crown (Ohio State Extension CFAES).

Harvested stalks store for 2–4 weeks at normal refrigerator temperatures and 95–100% relative humidity; perforated polyethylene bags in the crisper drawer give the best results. Rhubarb also freezes well — chop stalks and freeze in a single layer, then transfer to bags for long-term storage.

Dividing rhubarb crowns refreshes the planting, prevents overcrowding, and is the only reliable method of propagating named varieties. Divide crowns when the plant begins producing 25–30 or more smaller stalks instead of 12–18 larger ones — this overcrowding signal typically occurs around the fifth or sixth year (OSU Extension EC-797).

Divide crowns when plants are dormant — in the fall after leaves die back, or in early spring before growth begins. Each division must contain at least one large vegetative bud and a substantial piece of the root system (SDSU Extension; Iowa State Extension). Five- to six-year-old crowns should yield 8–10 good divisions, with the parent crown left with 3–5 buds (OSU Extension EC-797).

In the coldest climates (Zones 3–4), divide in spring — this gives the new divisions the full growing season to build reserves before winter. In milder climates, fall division is acceptable if it is late enough that the plant won't push new leaves before dormancy but early enough for roots to establish before the ground freezes. Replant divisions immediately — exposed crowns dry out quickly. If planting is delayed, store divisions in the refrigerator; rehydrate them before planting by soaking in water for at least two hours (Ohio State Extension CFAES).

Rhubarb plants periodically send up seed stalks — tall flowering shoots rising from the crown. Seed stalks draw energy from the plant and, if allowed to develop fully, can reduce bud formation and lower yields the following season (OSU Extension EC-797). Cut seed stalks off at the base as soon as they appear.

Bolting can be triggered by plant age, low soil fertility, alternating hot and cold temperatures, drought, or long nights. Some varieties form seed stalks more readily than others: Victoria and MacDonald produce them more often and at younger plant ages; Valentine and Canada Red are known for producing fewer seed stalks and doing so later in the plant's life (OSU Extension EC-797). If your planting consistently bolts heavily, variety selection at the next division is worth reconsidering.

If the plant is grown ornamentally rather than for stalk harvest, the tall seed stalks with their white flowers can be an attractive feature — but this is incompatible with maximum stalk production.