Artichokes for the Northeast: 2021 Data

Peyton Ginakes, University of Maine Cooperative Extension Research Associate
Mark Hutton, University of Maine Cooperative Extension Vegetable Specialist

Globe artichokes (Cynara cardunculus var. scolymus) are native to the Mediterranean region, which continues to dominate production globally. Monterey County, California produces the vast majority of US-grown artichokes. However, pockets of commercial artichoke production are developing in Texas and other regions.  In Maine and New England, artichokes can be found on farm stands and in backyard gardens.

Artichokes are perennial plants and members of the Asteraceae family, which also includes lettuce, thistles, chicories, and sunflowers. They are not reliably winter hardy in Maine and are instead grown as annuals from seed.  In order to produce buds in their first year, artichoke seedlings must undergo vernalization (exposure to cold temperatures). The harvestable component of artichokes are buds that, if left unharvested, will mature into vibrant purple inflorescences. Properly vernalized plants commonly produce 10-20 buds per plant, and occasionally more than this. However, only several buds per plant will be “primary” buds, which typically are large enough in diameter to market individually.  Buds that form at plant axials further down on the plant are “secondary” buds, which are edible but smaller in size.  The smaller buds comprise the majority of buds that a plant produces.

Seedling Production

In 2021, five globe artichoke cultivars were grown at the University of Maine’s Highmoor Farm in Monmouth, ME.  Cultivars included Colorado Star, Green Globe Improved, Imperial Star, Tavor, and Wonder. Artichokes were seeded on March 22. Two or three seeds were placed into 50-cell trays filled with Coast of Maine Bar Harbor Blend media. Trays were maintained on heated mats at 75-80 °F until seeds germinated, and then mat temperature was lowered to 65-70° F.  Trays were moved off heating mats 3 weeks after seeding.

thinned artichoke seedlings 3 weeks after seeding
Figure 1. Thinned artichoke seedlings 3 weeks after seeding.

Artichokes vary in their chill requirements, with most cultivars needing exposure to 250-500 hours below 50 °F. The recommended vernalization process for annual production involves exposing 4-6 leaf seedlings (approximately 6-8 weeks old) to an environment that is 45-50 °F for at least 10 days.  Many growers and researchers simply transplant artichokes early in the spring, relying on sufficiently cool temperatures to meet the chill requirements. While in many cases this can be successful, the process of vernalizing plants in coolers is not labor- or space-intensive and has the greatest chance of uniformly meeting chill requirements.

In this experiment, seedling trays were placed into a 40 °F walk-in cooler on May 10, for 14 days prior to transplanting.  Seedlings were thoroughly watered before vernalization and afterward were watered with Peter’s 20-20-20 (1 Tbsp per gallon) water-soluble fertilizer.

Table 1. Characteristics of artichoke cultivars evaluated in 2021 at Highmoor Farm in Monmouth, Maine.

Variety Color Days to Maturity Bred for Annual Production? Org Seed Available? Hybrid Status Spines
Colorado Star purple 75 Yes No OP Very
Green Globe Improved green 75 No No OP Yes
Imperial Star green 85 Yes Yes OP Yes
Tavor green 88 No Yes F1 No
Wonder green w/purple base 90 Yes Yes OP Few

Bed Preparation and Transplanting

Beds were prepared three days before transplanting by spreading 500 lb per acre of 10-10-10 fertilizer ahead of rototilling and forming raised beds with a single drip line.  Three mulch systems were evaluated: straw mulch, black plastic, and bare ground.  We hypothesized that 1) a straw mulch would mediate hot summer soil temperatures and reduce weed pressure, 2) black plastic would suppress weeds, but that elevated soil temperatures would affect yield, and 3) the bare ground treatment would not warm the soil but could allow for increased weed pressure.

The trial was transplanted on May 24 in a split-plot design, using mulch as the main plots and cultivar as the subplots, with four replicated blocks.  Therefore, each block had one bed of each mulch treatment, within which were five cultivar subplots.  Plots consisted of 12 plants with end plants used as guards.  Data were collected from the center ten plants of each plot.  Seedlings were transplanted using a jab transplanter in single rows at 24” in-row spacings.  Transplants were watered with Nutriculture Spoon-Feeding Soluble Fertilizer (12-45-10) at a rate of 3 lb per 50 gallons of water. Straw mulch was applied June 15.

Artichokes 65 days after transplanting.  Progression across the width of photo: bare ground, straw, and black plastic main plots.
Figure 2. Artichokes 65 days after transplanting. Left to right: bare ground, straw, and black plastic main plots.

Growing Conditions

The 2021 growing season can be characterized as changeable. The spring and early summer were warm and dry, while the fall was warm and wet. July was unusually wet and cool, and August more typical for this region.

Table 2. Recorded monthly weather data during the 2021 growing season at Highmoor Farm in Monmouth, Maine.

May June July August September October
Total rainfall (inches) 1.67 0.70 6.62 1.12 6.14 4.19
Avg daily temperature (°F) 56 69 66 72 64 54
Max daily temperature (°F) 92 94 87 92 81 79
Min daily temperature (°F) 35 46 50 52 46 37

Observed Pests and Diseases

Several insect pests negatively impacted artichoke production.  Aphids, primarily green peach aphid, fed on lower leaves, likely slowing growth early in the season.  Additionally, thrips caused twisting and curling of leaves and, to a lesser extent, some buds.  Tarnished plant bug was observed later in the season.  Applications of Asana XL were made on July 23, August 8, and September 8.  Gray mold (Botrytis cinerea) was the only identified pathogen in this trial, and caused only a minimal loss of buds.

Harvest

Artichoke harvest began August 20 and was continued weekly until September 17.  A final harvest was made October 1. To assess whether artichokes were ready for harvest (ie, fully sized), buds were squeezed to assess how densely packed the bracts were.  Buds were harvested by clipping with 2-3” of stem using hand pruners.  Buds were sorted into USDA size classes: 48s (3-3.5”), 36s (3.5-4”), 24s (4-4.5”) and 18s (>4.5”).  Buds smaller than 48s were categorized as “very small”.  Plants continued to produce high-quality buds (mostly secondaries) with some frost injury (dark spots on bracts) through light frosts until a hard freeze killed the plants.

Results

Transplant survival

The number of living plants in each plot two weeks after transplanting were recorded. Bare ground had the greatest plant survival followed by straw mulch and plastic mulched plots (Figure 3). The differences were not statistically significant.  However, there was an 11.5% decrease in plant survival observed on black plastic mulch compared to bare ground. Temperatures were very high at the time of transplanting, and plant loss is thought to be related to heat stress.  Transplanting under cooler conditions and irrigation are recommended for mitigating heat stress.

A bar graph showing the percentage (in increments of 25) of Plant Vigor plotted against three types of mulch: bare ground (between 75 and 100), plastic (just barely over 75), and straw (slightly higher than plastic, less than bare ground).
Figure 3. The proportion of plants that survived transplanting by mulch.

Vernalization

Success of vernalization was measured as the number of plants producing buds in each plot. Marketable buds were produced on 55% to 77% of plants, with no statistical differences between cultivars in the proportion of plants that produced buds (Figure 4). This may indicate that the chilling requirement was not fully met for each of the cultivars.  Alternatively, it may be that some seedlings of each cultivar were not at the proper developmental stage to respond to the vernalization treatment.

A bar chart that plots successful vernalization (in percentages of 25) plotted against five different varieties of artichokes: Colorado Star (75%), Green Globe Improved (74%), Imperial Star (65+/-%), Tavor (60+/-%), and Wonder (55+/-%).
Figure 4. Number of plants that produced buds by cultivar.

Yield Ranges

The sum of marketable yields over the season varied by variety, from single plants producing an average of about 1 to 1.6 lb and a maximum of about 3.3 to 4.8 lb.  Varieties with maximum yield potential are Green Globe Improved, followed by Tavor.

Table 3. Artichoke yield ranges on black plastic by cultivar at Highmoor Farm in Monmouth, Maine in 2021.

Marketable Yield per Plant (lb)
Cultivar Average Maximum
Colorado Star 1.46 3.40
Green Globe Improved 1.63 4.75
Imperial Star 1.12 3.32
Tavor 1.09 4.17
Wonder 0.98 4.09

Marketable Yield

Yield data are shown in Table 4 as both the number and weight of buds.  Buds under 3” in diameter could be sold either way, and are suitably sized for quart containers (or similar), depending on one’s market.  Any buds that fit into USDA size classes could likely be sold individually.  These size classes are summed in the table below and are categorized as “large”.

Green Globe Improved produced a greater number of very small and total buds than any other cultivar in this trial.  Imperial Star and Tavor were mid-level performers, both of which we found to be more attractive than Green Globe Improved (see next section for descriptions).

Table 4. Artichoke yield by cultivar and mulch at Highmoor Farm in Monmouth, Maine in 2021.

* Values in a column that are followed by the same letter are not different (p < 0.05).
Marketable Buds per Plant
Very Small
(< 3″)
Large
(3 to > 4.5”)
Total
Marketable
# g # g # g
Cultivar
Colorado Star 4.96 b* 305 0.82 172 5.79 b 478
Green Globe Improved 8.84 a 451 0.57 108 9.41 a 559
Imperial Star 5.54 b 304 0.77 157 6.31 ab 460
Tavor 5.82 b 338 0.71 146 6.54 ab 484
Wonder 5.35 b 284 0.51 98 5.87 b 381
Mulch
Bare Ground 6.50 360 0.87 a 176 a 7.37 535
Plastic 7.12 411 0.72 ab 141 ab 7.83 552
Straw 4.70 238 0.45 b 92 b 5.15 330

Artichokes on bare ground produced more large buds, by weight and number, than plants in the straw mulch.  While soil temperatures in the straw mulch treatment were moderated by the straw (data not shown), the mulch itself became a weed issue.  Clearing weeds from under the straw mat was difficult and often ineffective.  We suspect that this weed pressure caused a decline in yields relative to bare ground and plastic mulch treatments.

Small versus Large Bud Production

The proportion of total buds that were large enough to be marketed individually (> 3” diameter) was not statistically different across mulches or cultivars (Table 5).

Table 5. The proportion of individually marketable artichokes by cultivar and mulch in 2021 at Highmoor Farm in Monmouth, Maine.

% Buds > 3″ Diameter
Variety Bare Ground Plastic Straw
Colorado Star 36.6 34.4 34.6
Green Globe Improved 18.8 24.8 13.5
Imperial Star 37.5 32.5 50.2
Tavor 39.3 17.2 49.5
Wonder 38.0 18.9 18.0

Visual Observations

Colorado Star:

  • purple color dependent on cool temperature; early buds ranged in color from light purple to almost bronze
  • bud shape somewhat pointed and fairly consistent
  • bracts very pointed and spiny
  • bracts often tough/leathery

Imperial Star:

  • vigorous plants
  • fairly consistent bud shape within size classes; less consistent across size classes
  • smaller buds fairly compact but less so at larger sizes
  • fairly spiny bracts

Green Globe Improved:

  • vigorous plants, similar in stature to Imperial Star
  • inconsistent bud and bract shapes
  • occasional purpling at bract bases
  • tended to be less compact
  • fairly spiny bracts

Tavor:

  • vigorous plants
  • compact buds, consistently shaped in all size classes
  • rounded buds and bracts
  • few spines

Wonder:

  • bud shape varied from vase-like to conical
  • bracts were medium-compact
  • produced well until hard freeze (like Imperial Star and Tavor)
  • spineless

Acknowledgments

This work was supported by the University of Maine Agricultural and Forestry Experiment Station, Hatch Maine 021812, and the Maine Vegetable and Small Fruit Growers Association. High Mowing Seeds and Coast of Maine generously donated materials for this project.  We are grateful for assistance from David Handley, Greg Koller, Patricia McManus, and Pete Lugner, as well as field assistants Ethan Handley, Lydia Handley, Lee Lavoie, Brooke Martin, and Taylor Truman.

For any questions or comments on this research, please contact Peyton Ginakes at peyton.ginakes@maine.edu or Mark Hutton at mark.hutton@maine.edu or 207.933.2100.


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2.13.23

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