Organic Sweet Potato as a Commercial Crop in South Texas

Sweet Potato Production

Sweet potato originated in the tropical Americas and was grown extensively throughout the continent well before the arrival of the Europeans. It was then introduced to Europe, Africa, and Asia and is now one of the major root crops in the developing world where over 90% of the production is found. In the tropics, sweet potato can be grown as a perennial while in temperate regions it is grown as an annual.  

Mission, Texas, where the trial took place is in the USDA hardiness zone 9b.  Average first frost happens between December 21st through 31st, and the average last frost happens between January 21 though 31st.   The ecoregion is 34e, Lower Rio Grande Valley.  Heat zone days are 181-210 days over 86 degrees.

Sweet potato needs between 90 and 150 days of a frost free period to produce harvestable roots. In theory, the crop can be grown in south Texas starting in February all the way until November.  

Site Selection

Sweet potatoes grow best in well-drained, loamy soils which result in larger and well-shaped roots. Sweet potatoes grown in heavy clay soils, or in soils with high soil organic matter can result in rough, irregular roots.  

We chose organically certified sandy loam field just north of the Rio Grande River as the location of our trial.  Sweet potatoes do not require a lot of nitrogen fertility but do require significant potassium and phosphorus.  The field of our choice had very high amount of the potassium and phosphorus, and extremely low levels of nitrate.  The pH of the soil was 8.2, significantly higher than the recommended range of 6.0 to 6.8.  300 pounds per acre of sulfur was added to half of the plot as a soil amendment.

Source of water was the Rio Grande River.  The field was furrow irrigated, water pumped directly from the water source.

Site Preparation

Two-row disks were used to form raised beds.  8” ridges were created to improve drainage and root quality. Soil was plowed and disked to prepare a fine planting bed.  The beds were 40" wide.

Slips

Sweet potatoes are grown from slips.  Slips are shoots containing some roots which are pulled from growing plants. These slips had to be purchased from outside of Texas because there were no organic sweet potato slip available. 2000 slips each of Beauregard, Georgia Jet and Covington varieties were purchased from Steele Plant  Company in Tennessee. 100 additional Murasaki slips were ordered.

Beauregard was chosen for its promised highest yield potential.  In Kentucky yields have surpassed 500 boxes per acre.  Beauregard matures in 90 to 95 days.  It has copper-red skin with orange flesh, and is the most common variety grown.  

Georgia Jet promised great taste, but may crack, negatively affecting its yield potential.  It has bright  orange flesh and red skin. It promised maturity in 90 days.  

Covington is a variety developed by a scientist at North Carolina State University.  It promised highest quality but with lower yield potential than Beauregard.  Yield is estimated at 400 boxes per acre. It has rose colored skin with orange flesh.  It reaches maturity in 100 days.  

Muraski has purple skin and yellow flesh.  Very sweet and popular with the Asian population.  It promised maturity in 90 to 110 days.

The slips arrived in ventilated cardboard boxes, bunched and roots wrapped in wax paper.  The delivery from Tennessee to Mission, Texas took four days via UPS. 

Key Points:

• Organic slip supply is very low nationwide. 
• If you want to guarantee sourcing, you might have to get on the waiting list before the season starts.
• No slips are available during first three months of South Texas growing season Feb, Mar and April.

Spacing and Seeding

Once the slips arrived, they were separated into different varieties.  In the first planting, we used the slips straight from the box immediately after their arrival.  In the second planting, we dipped the slips in water for one hour.  The second method improved the standing significantly.  

Using a tomato transplanter, they were transplanted in a single row, 12” apart as recommended by the slip supplier.  

It is recommended that sweet potato slips are transplanted into rows at a depth of 3 inches with no less than 2 plant nodes in the ground and leaving at least 2 leaves or more above the ground.

Key Points:

• The slips should be refreshed by dipping the roots in few inches of water before transplanting.  If you are unable to transplant immediately, the slips should be able to hold their freshness if you keep them in one inch of water.
• Common local agricultural machinery was adequate for transplanting.  Literature kept talking about tobacco transplanter, so initially we were not sure if we could actually do it.  We used tomato transplanter we owned.

Irrigation

Once the slips were transplanted, the rows were irrigated through flood irrigation method. In our first planting, we started flooding the field once the entire field was planted.  In our second planting, we flooded each row as soon as the slips were transplanted.  The second method was much more successful for getting strong stands.

Proper irrigation improves slip establishment.

Irrigation of sweet potatoes need not be as frequent as for other vegetable crops, but is helpful during bulking stages. Excessive irrigation will cause splitting and cracking.  We applied one inch of water every ten days throughout the growing process.

Key Points:

• Because the soil is very hot at the time  of transplanting, it is better to fill the furrows as the rows get transplanted.  Do not wait until the whole field to finish if you have the option to start bringing water.
• One inch every ten days was be adequate.

Weed Management

Sweet potatoes are an aggressive crop that can quickly form a canopy, shading out weeds. Because our production was organic, we did not apply any herbicide but relied on manual cultivation.  Some of the weeds that infiltrated the fields are sunflowers and solanum elaeagnifolium  commonly known as silverleaf nightshade.  

5/5/2017

6/9/2017

6/23/2017

6/29/2017

Key Points:

• No need for herbicide.
• Hand weeding done only once.
• Sunflower and silverleaf nightshade showed up.

Insect Management

Pheromone insect traps were put out monthly to  collect data on insects that are above ground.  

No pesticide was used during the study.

Sweet potato weevil was the main cause of damage.

White fly was present but did not seem to affect production.

Key Points:

• Whitefly and sweetpotato weevils were present, the latter causing crop damage.  
• Further study is needed if pesticides or other organic methods can prevent or mitigate damage to crop.

Other Pests

The number one pest for the sweet potato for the tested sites were feral hogs.  Hoof marks were noticed once the plants became established.  The damage to the crop became apparent as the field matured.  The most amount of damage was done at harvest time when continuous rain in September rendered the fields too wet for people and machinery to enter.  The  hogs devastated the fields.

Key Points:

• Feral hogs are major threat to crop in test sites.

Harvest

In order to harvest the sweet potatoes, a potato digger was borrowed from another farmer.  Harvest was delayed due to weather conditions and availability of tractor and crew as the time frame coincided with our onion planting time.

One tractor was too weak to pull the digger against the friction the roots were creating.  We had to get a second tractor to help move it along.  The digger's bars kept snapping, especially when we were harvesting Murasaki variety.  The Murasaki failed to bulk up during our trial, and made long rope-like roots similar to burdock.  Georgia Jets grew faster than the other varieties.  Because we harvested everything at once, the Georgia Jets were over-grown.

The sizes were very irregular, even within the same variety.

The harvest was suddenly interrupted with a heavy rain that continued for the next two weeks, which made the fields too wet to enter. We gathered all we could under the circumstances, and retreated.

By the time the field dried, we found that feral hogs had done their damage.  The field that was partially harvested was completely run over.  The exposed sweet potatoes were all gone.  Few days later, a group of UTRGV students came to the field to help salvage what was left.  They were able to get some, but we could not collect data on yield.

Secondary Growth

Our trial field was destroyed by feral hogs.  

11/3/2017

We saw that there were some secondary growth happening from the remaining vines.  Having read that sweet potato was a perennial crop in the tropics, we decided to leave the field to see if we could get a crop by leaving them alone in south Texas.

11/3/2017

11/07/2017

11/7/2017

11/22/2017

Continuous exposure to the hogs, combined with multiple freezing incidents, made the field overrun with weeds.  Eventually the field browned completely.  The field was terminated at the end of February.

US parks and wildlife placed hog traps right at the location of our trial in order to study the migratory patterns of the hogs.  Apparently our field location was a hog hot spot.