Patricia Lazicki UCCE Vegetable Crops Advisor for Yolo, Solano, and Sacramento Counties

 

Walnut prices have greatly declined in recent years. As a result, we’re seeing walnut orchards pulled with the expectation that several thousand acres may be removed over the coming years. Some of that ground will be going into tomato.

There are some concerns about growing tomatoes on old walnut ground, but very little formal research exists in this area. To support grower decision-making in the absence of this information, I reached out to processors and growers for their thoughts on how to make the transition successfully.

Processors’ concerns

Old walnut ground presents some special challenges for tomato harvest. I spoke with representatives from several canneries to get their thoughts on what to be aware of, and tips for avoiding expensive problems.

What are some of the issues you’ve seen with harvesting and processing tomatoes that have come out of walnut: Woody material can both slow down the harvest and end up in the load. Canneries have zero tolerance for woody materials, as they can damage processing equipment, plug sieves, and can cause plant shut-down for cleaning. Nuts in the load will also cause it to be rejected, as potential allergens. Roots are especially an issue, since a field may look clean but have many roots below the surface that will be brought up by the harvester. Roots may also not be recognized by the dirt sorters. If detected as MOT (material other than tomato), large wood chunks mean a big deduction for the grower. Wood or nuts not showing up in the Processing Tomato Advisory Board (PTAB) sample but detected while the load is being dumped can lead to the load not being processed. As well as being a financial hit for the grower ($138/ton*26-ton trailer= $3,588 if reconditioning isn’t an option), rejected loads also mean more scrutiny in the future.

How does walnut differ from almond or other crops that leave woody residue? Sunflower, corn, and tomatoes can also leave woody residues in the field. However, woody orchard debris break down more slowly in soil. It can also be less likely to float, making it harder for the processor to sort it out from a load. Walnut orchards are more challenging than almonds as roots can be larger (a 40-year-old orchard can have roots that are 12 feet long and 8 inches in diameter). Large pieces are especially dangerous as they’re the most liable to break equipment. They also take more labor to remove and persist longer in the soil.

Have you worked with any fields where the biomass has been chipped and then returned to the soil? Are there any special considerations for these fields?

This practice isn’t common, so no specific advice. It would probably depend on how finely the material is chipped. Likely, if chips end up on the harvester they will be more difficult to sort out than roots and more likely to migrate to the top of the load, therefore they are more likely to show up as MOT.

Any tips for growers to avoid costly penalties?

• Consult first with the processor. Consider growing another crop before putting in tomato.
• Due diligence in root removal. The more labor put in on the front end, the cleaner the loads will be at harvest.
• If tomato is the first crop after a walnut orchard, consider hiring extra sorters to help prevent woody material from entering the load.

 

Grower Experience

Bullseye Farms is a large Yolo County operation that has experience successfully transitioning fields from walnut to tomato. Their take is that it’s expensive and laborious to clean the ground well, but they haven’t had problems with the harvest. Higher yields (likely due to low disease pressure) will make it profitable over time.

How do you remove woody materials from the fields? Push the trees over, grind them up, and haul the biomass off. After this, run a ripper through to 2.5 feet and have hand crews pick up the roots, repeat until the field is clean. It’s a significant cost; about $850/acre in labor on top of the cost of the ripping, grinding, and hauling ($1400-$1500/acre). The older the orchard is, the more laborious it will be to remove the roots.

What are some issues to watch for when transitioning ground from walnut to tomato?

• Pre-emergent herbicides used in orchards can have plant-back restriction periods of up to 18-20 months; it’s important to check the dates and products used.
• Nutrient tie-up hasn’t been a problem when the field was well cleaned and biomass was removed. Fertility needs haven’t differed so far from those of other fields. However, tie-up will likely be more of an issue if chipped biomass is returned to the field.

Any issues that you have had or would foresee where the biomass has been chipped and incorporated? So far, all biomass has been removed. In one field, piles of chips from an orchard that was ripped out in January sat from June to October before removal. In a tomato crop planted the following April, there were poorly performing patches in the areas where the piles had been placed. However, it was unclear if this was due to allelopathy (live walnut trees produce a chemical called juglone that has a negative effect on tomatoes planted near them), nutrient tie-up, or some other cause. The Yolo-Solano Air Quality District Agricultural Chipping Program is offering monetary incentives to use chips on-farm, and there are plans to experiment next year on a limited scale.

How have you seen this transition be most successful? Just put in the labor to really get the roots out and be cautious about incorporating materials.

Take-home points

Tomatoes have been successfully grown directly following walnut.

There are risks, and it’s important to put in the work after orchard removal to avoid problems at harvest.

If planning to follow walnut with tomato, it could be a good idea to discuss with the processor how you plan to clean the field.

Allowing a transition period of 1-2 years before planting tomato in old walnut ground will reduce the associated risks.

For more information, please contact Patricia Lazicki at 530-219-5198 or palazicki@ucanr.edu