Summary: This 23-year experiment looks at how orange trees grow in a multiple-species cover crop of 29 native weeds and 60 companion plants. A small grove of 27 trees is followed from crop year 2000 (before citrus greening) through 2022. (Citrus greening appeared in Florida around 2005). Live tree number dropped from 27 to 24. Average early season fruit Brix dropped from 14.6 to 11.5%. Mean late season Brix declined from 16.0 to 12.9% Average fruits per tree decreased from 200 to 140. Average market fruit weight dropped from 7.33 to 4.5 ounces. Mixed species cover crops are not a cure-all for citrus greening, but they enable orange trees to live with the disease and produce fruit of acceptable quality.

Experimental Location: Venus, Highlands County, Florida, United States of America. Zip Code: 33960. 27.0670 degrees North Latitude. 81.3571 degrees West Longitude.

Climate: Venus has a humid subtropical climate with a distinct monsoon season. The wet months are June through September (31 inches of rain). The dry months are November through January (6 inches of rain). Elevation = 82 feet above sea level. Average Annual Temperature = 72.5 degrees Fahrenheit. The warmest month is July (High = 92 degrees Fahrenheit). The coldest month is January (Low = 48 degrees Fahrenheit). Average Annual Rainfall = 52 inches. Average First Frost (36 degrees Fahrenheit) = 21 December. Average Last Frost in Spring (36 degrees Fahrenheit) = 20 February. Frost Free Growing Season = 317 days = approximately 10 months. Note: Venus is the warmest area in Florida. Frosts are rare and unpredictable.

Experimental Design: A small grove of 27 trees is monitored over 23 years for longevity (age), fruit quality (percent sugar content), and yield (fruit number and weight). Sugar content in degrees Brix is measured with a refractometer by sampling juice of 10 early season and 10 late season fruits from each tree. (1 degree Brix = 1 percent sugar content by weight = 1.5 ounces of sugar in 1 gallon of pure water). 23 years of data ensure reliable averages for comparison with commercial orchards.

Soil Type: Sandy Gravel. South Florida soils are deficient in most primary nutrients and trace elements. The best management practice for these problem soils is to keep fields “green” all year long. Growing plants add organic matter to the soil and live roots feed sugar to beneficial micro-organisms.

Rotation: Experimental orchard was planted in 1970 on pasture seeded with multiple species forages. It is good practice to plant citrus on fresh ground = hay fields or meadows 7 or 8 years old. Long rotations cure “orchard syndrome” = buildup of nematodes and diseases that make trees sick and cut yields by 30 to 50 percent.

Variety: Citrus sinensis cultivated variety “Cyprus”. This is a “vintage” variety imported from the island of Cyprus in 1880. Cyprus oranges are large and sweet, about the size of a grapefruit = 13 to 18 ounces and 14 to 20 degrees Brix. Cyprus oranges grown in Florida are smaller (7 to 14 ounces) and contain less sugar (14 to 16 degrees Brix for late season fruit).

Spacing: Experimental trees are plated 30 feet equidistantly = 48 trees per acre to promote maximum airflow through the orchard and optimum light penetration into the canopy. For comparison, most modern Florida orchards have 70 trees per acre spaced 25 feet apart.

Cultivation: Experimental orchard was not plowed, disked, harrowed, or cultivated. Weeds and cover crops were allowed to grow without interference. Orchard was mowed once yearly just before harvest.

Agronomy Note: Conventional tillage of South Florida soils is a pointless waste of effort. Plowing sandy gravels merely stirs up more rocks. Cultivation destroys the network of beneficial fungi upon which orange trees depend for water and minerals.

Fertilizer: No nitrogen fertilizers (chemical or organic) were used in this experiment. Legumes and independent soil bacteria supplied nitrogen for orange trees. 1 ton each of clay, phosphate rock, and greensand were broadcast yearly over every acre. 500 pounds of fritted trace elements were applied to each acre every 5th year.

Agronomy Notes: Small amounts of clay, about 3/4 ounce per square foot yearly, help sandy soils hold water and nutrients for better plant growth. Only modest amounts of clay are needed to “strengthen” sandy fields, never more than 10 percent by weight in the top 6 inches of soil. Adding excess clay reduces porosity and makes soil too sticky.

Micro-nutrients are mixed with molten soft glass then crushed into coarse sand called “frit”. Minerals are released slowly as the glass dissolves in the soil.

Herbicide: No herbicides were used in this experiment. Weeds and cover crops were allowed to grow with “wild abandon” to encourage maximum populations of beneficial insects. The trees did not seem to mind competition from their companions even when occasional vines climbed into the branches. Errant vines were removed by hand and the orchard mowed immediately before harvest.

Fungicide: No fungicides were used in this trial. Widely spaced trees grown in weeds are remarkably healthy even when infected by citrus greening.

Insecticide: No insecticides (synthetic or natural) have ever been used in this orchard. One of the significant advantages of growing citrus with many companion plants is that trees do not require spraying. Large populations of native predators and parasites keep pests below economic threshold levels.

Irrigation: Experimental trees are irrigated by overhead sprinklers installed when the orchard was planted in 1970. Trees receive 2 inches of water weekly, as needed.

Harvest: Experimental trees were harvested by hand. All attempts to use robots have failed miserably. Machine intelligence and artificial vision systems cannot handle random limbs and branches. No robot has ever picked more than 60% of available fruit even when citrus trees were espaliered and trained on wires. Machine harvest of oranges remains a faint hope far in the future.

60-Species Cover Crop with 48% Legumes:

29 legumes (48.33%) + 15 forbs (25%) + 11 grasses (18.33%) + 5 root crops (8.33%) = 60 total species (99.99%).

We plant a little bit of whatever cover crop seeds are available. Our goal is broad genetic diversity. Wide variety supports the maximum number of beneficial insects and micro-organisms. Admittedly, this is “black box” science. We do not know how the biology works but orange trees somehow manage to survive and ripen good fruit. Cover crops are not a panacea for citrus greening but sowing seeds is far less costly than alternative treatments.

Alfalfa = Lucerne = Medicago sativa

Birdsfoot Trefoil = Lotus corniculatus

Black Medic = Medicago lupulina

Buckwheat = Fagopyrum esculentum

Butterfly Pea = Centrosema rotundifolium

Canary Seed = Phalaris canariensis

Caraway = Carum carvi

Centro = Centrosema pubescens = Centrosema molle

Clitoria = Clitoria ternatea

Cowpea = Vigna unguiculata

Crimson Clover = Trifolium incarnatum

Deer Vetch = Aeschynomene americana

Dill = Anethum graveolens

Dutch White Clover = Trifolium repens

Egyptian Clover = Trifolium alexandrinum

Fenugreek = Trigonella foenum-graecum

Flax Seed = Linum usitatissimum

Forage Kale = Brassica oleracea sabellica

Forage Maize = Zea mays

Forage Pea = Pisum sativum

Forage Sorghum = Sorghum bicolor x Sorghum sudanense

Forage Soybean = Glycine max = Glycine soja

Forage Turnip = Brassica campestris rapa

Frost Bean = Vicia faba minor

Garden Radish = Raphanus sativus

Grain Sorghum = Milo = Sorghum bicolor

Hairy Indigo = Indigofera hirsuta

Jack Bean = Canavalia ensiformis

Lupine (blue) = Lupinus polyphyllus

Mexican Sunflower = Tithonia rotundifolia

Mung Bean = Green Gram = Vigna radiata

Oat = Avena sativa

Okra = Abelmoschus esculentus

Partridge Pea = Cassia rotundifolia = Chamaecrista species

Pearl Millet = Pennisetum glaucum

Phacelia = Phacelia tanacetifolia

Pigeon Pea = Cajanus cajan

Plantain = Plantago major

Rape Seed = Rape Seed = Brassica napus

Red Clover = Trifolium pratense

Red Lentil = Lens culinaris

Rice (African) = Oryza glaberrima

Rice (Indian) = Oryza sativa indica

Rice (Oriental) = Oryza sativa japonica

Rutabaga = Brassica napus napobrassica

Safflower = Carthamus tinctorius

Scarlet Runner Bean = Phaseolus coccineus

Sesame = Sesamum indicum

Sesbania = Sesbania exaltata

Stock Beet = Magelwurzel = Beta vulgaris

Sugar Beet = Beta vulgaris saccharum

Sunflower = Helianthus annuus

Sunn Hemp = Crotalaria juncea

Sweet Clover = Meliotus officinalis

Sweet Sorghum = Sorghum bicolor saccharum

Velvet Bean = Mucuna pruriens

Winter Barley = Hordeum vulgare

Winter Rye = Secale cereale

Winter Vetch = Vicia villosa

Winter Wheat = Triticum aestivum

Agronomy Notes: Cover crop seeds may be mixed with corn meal, weed seed meal, or similar carrier for more even distribution. Use a no-till drill and sow not less than 20 pounds per acre in 2-inch-deep furrows spaced 7 inches apart. Alternatively, mow field first, broadcast seed, then make 1 pass only with a rear-tine rototiller set 2 inches deep. Cover crops may also be surface seeded = Sow-and-Mow: Broadcast seed into standing weeds 5 to 6 feet tall then immediately mow to cover and protect seed. Prompt and frequent irrigation is essential to speed germination of surface sown seeds.

Sesbania exaltata is a useless weed to many South Florida growers. All varieties of sesbania are legumes that fix 130 pounds of nitrogen per acre every 60 days when grown in monocultures. The difference between a “weed” and a useful plant is often careful management.

Common Agricultural Weeds in Highland County:

Following is a list of 28 weed species most common on South Florida farms. These are only the most bothersome weeds. There are many other species in fields around Venus. Subtropical warmth encourages rapid plant growth so weed competition is constant on agricultural fields. The only biologically effective way to manage weeds is to keep ground covered with live crops year-round. Shade and intense competition prevent weeds from becoming established.

American Black Nightshade = Solanum americanum

Annual Ryegrass = Lolium multiflorum

Bristly Starbur = Acanthospermum hispidium

Bull Thistle = Cirsium vulgare

Cocklebur = Xanthium strumarium

Coffee Senna = Senna occidentalis

Crabgrass = Digitaria sanguinalis

Fall Panicum = Panicum dichotomiflorum

Florida Beggarweed = Desmodium tortuosum

Goosegrass = Eleusine indica

Hairy Nightshade = Solanum physalifolium

Johnsongrass = Sorghum halepense

Lambsquarters = Chenopodium album

Morning Glory = Ipomoea purpurea

Palmer Amaranth = Amaranthus palmeri

Pigweed = Amaranthus blitum

Purple Nutsedge = Cyperus rotundus

Purselane = Purslane = Portulaca oleracea

Ragweed = Ambrosia artemisifolia

Ragweed Parthenium = Parthenium hysterophorus

Redroot Pigweed = Amaranthus retroflexus

Sandbur = Cenchrus echinatus

Sickle Pod = Senna obtusifolia

Smooth Pigweed = Amaranthus hybridus

Spanish Needle = Bidens alba

Spiny Amaranth = Amaranthus spinosus

Texas Panicum = Urochloa texana

Yellow Nutsedge = Cyperus esculentus

Experimental Measurement: Harvested fruits were weighed on a digital platform scale accurate to 1/100th pound. All numbers are rounded down to the nearest 1/10th pound.

Summary of Experimental Results:

Effect of Citrus Tree Greening on Orange Tree Growth

Trees spaced 30 x 30 feet apart, planted in 1970. Cyperus Orange grafted on Trifoliate Orange rootstock. Irrigated orchard covered with native weeds and 60-species companion plant mix mowed only before harvest. 1 ton each of clay, phosphate rock, and greensand broadcast per acre yearly. 500 pounds per acre of fritted trace elements applied every 5 years.

Harvest Year 2000

Tree Number = 27

Early Season Fruit Brix = 14.1%

Late Season Fruity Brix = 16.0%

Fruits per Tree = 200

Market Fruit Weight = 7.3 oz

Yield per Tree = 91.2 lb

Note: Degrees Brix is a measure of “soluble solids” = sugar content. 1 degree Brix = 1 percent sugar.

Harvest Year 2022

Tree Number = 24

Early Season Fruit Brix = 11.5%

Late Season Fruit Brix = 12.9%

Fruits per Tree = 140

Market Fruit Weight = 4.5 oz

Yield per Tree = 39.3 lb

Commentary: 39 pounds of fruit per tree is nothing to cheer about unless the alternative is bulldozing an entire orchard. Growing orange trees in “weeds” has 3 distinct advantages:

(1) Companion plants help rejuvenate trees so they have productive lives exceeding 50 years. Most trees infected with citrus greening live only 15 years or less.

(2) Multiple species cover crops enable orange trees to ripen fruits with high sugar contents, 11.5 to 12.9 degrees Brix. These values compare favorably with oranges from California where citrus greening has not yet spread.

(3) Trees grown in weeds do not have economically significant insect problems. Experimental trees remain unsprayed after 54 years. Companion plants provide food, shelter, and alternate hosts for beneficial predators and parasites. The good bugs eat the bad bugs.

Little is known about the interaction between species in a cover crop mix. Even less is known about the interrelationships between myriad species of soil micro-organism and plant roots. What is known is that a lively trade exists between plants and symbiotic fungi. Roots provide sugar to fungi in exchange for water and minerals. Perhaps complex organic compounds are traded as well. Fungi make anti-biotics to protect themselves from bacteria. Citrus greening is a bacterial disease. Could anti-bacterial chemicals be responsible for keeping orange trees healthy?

Related Publications: Biological Agriculture in Temperate Climates; Crops Among the Weeds; Living Mulches for Weed Control; Managing Weeds as Cover Crops; The Twelve Apostles (multi-species cover crop); Trash Farming; Weed Seed Meal Fertilizer.

Would You Like to Know More? For more information on biological agriculture and multi-species cover crops please visit: http://www.worldagriculturesolutions.com — or — mail your questions to: Eric Koperek, Editor, World Agriculture Solutions, 413 Cedar Drive, Moon Township, Pennsylvania 15108 United States of America — or — send an e-mail to: worldagriculturesolutions@gmail.com.

About the Author: Mr. Koperek is a plant breeder who farms in Pennsylvania during summer and Florida over winter. (Growing 2 generations yearly speeds development of new crop varieties).

Index Terms: Biological Agriculture; Biological Insect Control; Candidatus liberibacter asiaticus (citrus greening bacteria); Companion Plants; Citrus Greening (disease); Cyprus Orange (Citrus sinensis cultivated variety Cyprus); Huanglongbing (citrus greening disease); Multiple Species Cover Crops; Orange Fruit Brix; Orchard Floor Management; Sweet Orange (Citrus sinensis); Trifoliate Orange Root Stock (Citrus trifoliata); Weeds (as cover crop).

Publication Date: June 2023, Homestead, Florida.