Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: organic

Use of Mulch in Organic Orchards Called into Question

Because of potential contamination of mulches from pesticides, mulch use is being called into question and testing may be required for mulches applied to organic orchards.  This is a recent ruling.  As always, anything applied to a certified organic operation should be cleared with the certifier first.


Posted on Wednesday, June 29, 2016 at 6:23 AM
Tags: certified organic (1), compost (7), composting (2), farming (4), green gold (1), green waste (2), greenwaste (2), mulch (15), organic (12), pesticides (26)

Organic Nutrient Management in California Is NOT the Same as Other Places in the US

Nutrient availability from organic sources has been considered “slow release” by many growers and advisers.  This may be true in environments are colder and especially soils are cooler.  Organic nutrients are dependent on microbes to break down materials and release those nutrients, and when soils are cold, microbes can't do their thing.  Soils in much of agricultural California tend to be warm and lack the freezing conditions that occur in many soils in the continental US.  Imagine how much microbial activity occurs in the Mid-West when soils cool down to 32 deg F at a four inch depth and deeper.  The top layers of soil are where organic matter accumulates and where most microbial activity occurs.  When soils cool below 50 deg F, nitrogen leaching becomes less common, because less activity is occurring which also coincides with much less plant growth.

Soils in coastal California rarely fall below 50 deg F in the surface layers, so microbial activity is ongoing, all year long.  So the question is, how “slow acting” are organic fertilizers?  A recent study by Tim Hartz, Richard Smith and Mark Gaskell looked at release rates of injectable organic fertilizer and found that much of the nutrient release occurs within about a week after application depending on the formulation and temperature during the study.  The results conform to another study that they did where they evaluated the nitrogen release rates of dry formulations of organic fertilizers – compost, manures, feather meal, etc.

Aside from the issues of the higher costs of these materials and their potential clogging, there is the issue of application timing.  In the case of avocados and citrus, adequate levels of nitrogen are needed in the trees going into to fruit set in order to optimize set.  And then after fruit set, in order to maintain growth into the fast growth period, again nitrogen needs to be adequate.  Using organic fertilizers with a rapid conversion to useable forms of nitrogen, means that application timing should coincide with these critical periods in tree phenology or growth cycle.

Using information on organic nutrient management based on work from cold soil climates needs to be carefully evaluated before applying it to California soils.  One of the most common problems in organic production is nitrogen management. Part of the problem is the cost of supplemental nitrogen amendments, but also learning to anticipate when that applied nutrient becomes available to the plant.  Developing better estimates for local release rates and patterns will better help manage organic nutrient sources. 

Read more:

Nitrogen Availability from Liquid Organic Fertilizers by T.K. Hartz, R. Smith and M. Gaskell

Summary: Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates additional N fertilization. The increasing use of drip irrigation has created a demand for liquid organic fertilizers that can be applied with irrigation. The N availability of three liquid organic fertilizers was evaluated in an incubation study and a greenhouse bioassay. Phytamin 801 contained fishery wastes and seabird guano, while Phytamin 421 and Biolyzer were formulated from plant materials. The fertilizers ranged from 26 to 60 g·kg−1 N, 8% to 21% of which was associated with particulate matter large enough to potentially be removed by drip irrigation system filtration. The fertilizers were incubated aerobically in two organically managed soils at constant moisture at 15 and 25 °C, and sampled for mineral N concentration after 1, 2, and 4 weeks. In the greenhouse study, these fertilizers and an inorganic fertilizer (ammonium sulfate) were applied to pots of the two organically managed soils with established fescue (Festuca arundinacea) turf; the N content of clippings was compared with that from unfertilized pots after 2 and 4 weeks of growth. Across soils and incubation temperatures, the N availability from Phytamin 801 ranged from 79% to 93% of the initial N content after 1 week, and 83% to 99% after 4 weeks. The plant-based fertilizers had significantly lower N availability, but after 4 weeks, had 48% to 92% of initial N in mineral form. Soil and incubation temperature had modest but significant effects on fertilizer N availability. Nitrification was rapid, with >90% of mineral N in nitrate form after 1 week of incubation at 25 °C, or 2 weeks at 15 °C. N recovery in fescue clippings 4 weeks after application averaged 60%, 38%, and 36% of initial N content for Phytamin 801, Phytamin 421, and Biolyzer, respectively, equivalent to or better than the N recovery from ammonium sulfate.



fertilizer injector
fertilizer injector

Posted on Saturday, March 19, 2016 at 8:59 AM
Tags: avocado (289), biolyzer (1), citrus (338), fertigation (2), fertilizer (14), fertilizer injection (1), fish emulsion (1), guano (1), liquid fertilizer (1), nitrogen (10), organic (12), phytamin (1), soy bean (1), soya (1)

An Organic Herbicide that Seems to Work

When Richard Smith tells you that he is impressed with efficacy of an organic herbicide you better take notice. Richard shared his results on this blog site last year and showed good weed control with 'Suppress' from Westbridge. This OMRI approved herbicide is a mix of caprylic and capric acids and is a contact material that interferes with plant cells membranes causing leakage and desiccation.

It looked like a good fit for organic plasticulture systems such as strawberry that have wet weedy furrows which are difficult to access with mechanical tools because of proximity to plastic. We placed a trial in a very weedy field that also had one of the SoCal classics–yellow nutsedge. 'Suppress' at 6 and 9% by volume was applied to well-established weeds just before strawberry planting.

The effects of application were noticeable in minutes. We learned that:

  • Both 6 and 9% rates provided nearly 100% control of common lambsquarter (predominant species) and other occasional broadleaved weeds.

  • About 10-15% of common purslane plants survived and yellow nutsedge seemed unaffected by application. However, biomass of both purslane and nutsedge were significantly reduced, suggesting that production of seed and tubers for the two weed species, respectively, may be delayed.

  • Good coverage was important and some of the horizontally–inclined leaves of broadleaf weeds likely intercepted the herbicide deposition to vertically inclined nutsedge shoots.

  • When we simulated drift by over-spraying strawberry, it responded just like any broadleaf perennial plant—the canopy wilted and dried but in 3 weeks the new leaves developed from the crown. This was also true for the neighboring bindweed that lost above ground canopy but had new growth within a month after 'Suppress' application.

  • Since perennial weeds or those in soil seedbank are not controlled, repeated applications are needed with obvious caution of avoiding the spray drift to the crop.


Posted on Monday, February 22, 2016 at 8:23 AM
Tags: avocado (289), citrus (338), herbicide (11), organic (12), weeds (32)

Organic Herbicides in Orchards


Using organic herbicides in production fields and non-crop areas.

The forecasts call for rainy winter and that means a lot of weeds. During dry times perennial weeds tend to grow better than annual weeds, since perennial structures such as underground rhizomes or tubers can support them and give competitive advantage. Seed of annual weeds in dry soil may have been losing viability, senescing or eaten during this time, but many have remained dormant and look forward to the wet winter us much as the rest of us.

Controlling weeds ‘organically' is always an extra challenge whether you are in a certified field or in an area where synthetic herbicides are not desired. Hand-weeding, already expensive, is even a greater burden with limited labor availability, and frankly not much fun either. Of course sanitation and prevention, mechanical and cultural management are essential in organic systems. That requires time and commitment and can quickly become your not-so-favorite pastime.

Organic herbicides have traditionally been contact materials with no systemic activity. This means that they only affect tissue that they contact and do not translocate through the plant like most synthetic herbicides. Thus, good coverage is critical for these contact materials. Many years ago the first herbicides were sulfuric acid and diesel fuel, current organic materials are often acids or oils too, although a lot more benign.

Recent trials by the University of California weed scientists showed that several organic herbicides provided decent control of easy to control pigweed and nightshade when they were small. When weeds were 12 days old, a mixture of 45% clove and 45% cinnamon oil, 20%-acetic acid and d-limonene gave 61-89% control; however only d-limonene controlled 19-day old weeds and none was effective on one-month old ones. As weeds get bigger they also develop a protective cuticle that minimizes efficacy of these herbicides.

This year we conducted trials with a recently OMRI approved herbicide for row crops, trees and vines that is a mix of caprylic and capric acids. It disrupts cell membranes of plans and causes the contents to leak and plants to desiccate. It worked well at 6 to 9% by volume mixture with water and gave 90% control of little mallow and >95% of annual sowthistle compared to untreated checks. We have also tested it in organic strawberry furrows before planting the crop to prevent potential injury from drift. Furrow cultivation does not get close to the plastic mulch that covers the beds to prevent tears, so the weeds in that zone are good target for the herbicide. This fatty acid herbicide provided excellent control of common lambsquarter, reduced the growth of common purslane but didn't do much for yellow nutsedge - one of our notoriously difficult to control perennial weeds (Figure). The bigger weeds need higher rates (9% is the maximum labeled rate) and better coverage. When you have multiple layers of weed leaf canopy and diverse architecture some plants or their parts may be protected by others that intercept the deposition of the herbicide. When on target, this contact material acts fast – you can see results within 2-3 days, however, it does nothing to weed propagules in soil and has no residual activity against wind-dispersed weed seed that fly in after application. This means the control does not last and you will need additional applications or other control measures. Repeated application is not a problem in a non-crop area and is a great way to deplete your weed seedbank, but crop protection from drift, such as shielded sprayers, is necessary to avoid off target plant injury.


Figure. Weed control in strawberry furrows prior to planting with 9% by volume of fatty acid herbicide (top) and weeds in untreated check (bottom)

organic herbicide dead
organic herbicide dead

organic herbicide live
organic herbicide live

Posted on Wednesday, November 25, 2015 at 9:04 AM
Tags: acids (1), amaranth (2), avocado (289), cheeseweed (1), citrus (338), herbicides (18), mallow (1), malva (2), orchards (7), organic (12), pigweed (2), weeds (32)

Organic Herbicides: do they work?

In recent years, several organic herbicide products have appeared on the market.  These include Weed Pharm (20% acetic acid), C-Cide (5% citric acid), GreenMatch (55% d-limonene), Matratec (50% clove oil), WeedZap (45% clove oil + 45% cinnamon oil), and GreenMatch EX (50% lemongrass oil), among others.  These products are all contact-type herbicides and will damage any green vegetation they contact, though they are safe as directed sprays against woody stems and trunks.  These herbicides kill weeds that have emerged, but have no residual activity on those emerging subsequently.    Additionally, these herbicides can burn back the tops of perennial weeds, but perennial weeds recover quickly. 

These products are effective in controlling weeds when the weeds are small and the environmental conditions are optimum.  In a recent study, we found that weeds in the cotyledon or first true leaf stage were much easier to control than older weeds.  Broadleaf weeds were also found to be easier to control than grasses, possibly due to the location of the growing point (at or below the soil surface for grasses), or the orientation of the leaves (horizontal for most broadleaf weeds).

Organic herbicides only kill contacted tissue; thus, good coverage is essential.  In test comparing various spray volumes and product concentrations, we found that high concentrations at low spray volumes (20% concentration in 35 gallons per acre) were less effective than lower concentrations at high spray volumes (10% concentration in 70 gallons per acre).   Applying these materials through a green sprayer (only living plants are treated), can reduce the amount of material and the overall cost

Adding an organically acceptable adjuvant has resulted in improved control.  Among the organic adjuvants tested thus far, Natural wet, Nu Film P, Nu Film 17, and Silwet ECO spreader have performed the best.  The Silwet ECO spreader is an organic silicone adjuvant which works very well on most broadleaf weeds, but tends to roll off of grass weeds.  The Natural wet, Nu Film 17 and Nu Film P work well for both broadleaf and grass weeds.  Although the recommended rates of these adjuvants is 0.25 % v/v, we have found that increasing the adjuvant concentration up to 1% v/v often leads to improved weed control, possibly due to better coverage.  Work continues in this area, as manufacturers continue to develop more organic adjuvants. Because organic herbicides lack residual activity, repeat applications will be needed to control new flushes of weeds.  


Temperature and sunlight have both been suggested as factors affecting organic herbicide efficacy.  In several field studies, we have observed that organic herbicides work better when temperatures are above 75F.   Weed Pharm (acetic acid) is the exception, working well at temperatures as low as 55F.  Sunlight has also been suggested as an important factor for effective weed control.   Anecdotal reports indicate that control is better in full sunlight.  However, in a greenhouse test using shade cloth to block 70% of the light, it was found that weed control with WeedZap improved in shaded conditions.  The greenhouse temperature was around 80F.  It may be that under warm temperatures, sunlight is less of a factor.


Organic herbicides are expensive at this time and may not be affordable for commercial crop production.   Because these materials lack residual activity, repeat applications will be needed to control perennial weeds or new flushes of weed seedlings.   Finally, approval by one's organic certifier should also be checked in advance as use of such alternative herbicides is not cleared by all agencies.


Posted on Thursday, October 15, 2015 at 8:42 AM
  • Author: Tom Lanini
Tags: herbicides (18), organic (12), vegetation management (2), weeds (32)

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