sustainable agriculture

Soil-biodegradable plastic mulch: Is it right for you?

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By Shuresh Ghimire, UConn Cooperative Extension and Andy Radin, URI Cooperative Extension

Something important for you to consider: your use of plastic mulch in vegetable production. This is especially worthy to think about given that plastic mulch isn’t just for tomatoes, peppers, and eggplant anymore. Most small-scale growers use it for onions, lettuce, herbs, and more. You’ve no doubt heard about biodegradable mulch, but possibly not so great things. Here are some important questions you may have, and some real answers.

  1. Can plastic mulch actually “degrade?” Yes, and No.

First, the NO.

Degradation caused by sunlight, heat, moisture, and mechanical stress results in ever-smaller fragments of plastic, ultimately becoming what are called “microplastics.” According to a quick search on Google Trends, this word as a topic has increased dramatically over the last 5 years, which corresponds to an increase in our understanding of just how pervasive these pollutants are on land and in the oceans, and the possible and probable effects they have on species richness and diversity. It’s a disturbing and inescapable fact that plastics are changing the planet for the worse.

Now for the YES!

Degradation of plastic by microorganisms, known as biodegradation, is very much a real thing, provided the plastic is made of the polymers that microbes can consume. Soil-biodegradable plastic mulch breaks down into CO2, water, and microbial biomass.

But let it be stated right here up front: except for paper-based mulches, there are no biodegradable plastic mulches that are approved for use by certified organic farms in the United States. (See Box with NOP standards for reasons why.) If you are a certified organic grower, you cannot make the switch. You are permitted to use paper-based mulches, as long as they do not contain any synthetic materials. However, as they reduce the soil temperature, these do not perform well in our cold springs and early summers when warmth is desired. Further, our (usually) plentiful rainfall and warm summer weather causes rapid breakdown of these materials.

But if you otherwise do use plastic mulch, please read on as we consider a list of potential issues with the use of soil-biodegradable mulch (BDM.)

  1. How much do BDMs break down during the growing season?

Ghimire et al. (2018) found that among 4 products they tested for pumpkin production for two years in Mount Vernon, WA and Knoxville, TN, by the end of the season, less than 8% of the soil originally covered became exposed in Mount Vernon, and that was less than 25% in Knoxville. At Mount Vernon, the 2-year average daily air temperature for the pumpkin growing season (June–September) was 63 °F, RH was 76%, and total rainfall was 6 inches (4 inches in year 1 and 8 inches in year 2). At Knoxville, the 2-year average daily air temperature was 77 °F, RH was 80%, and total rainfall was 10 inches (14 inches in year 1 and 5 inches in year 2). These mulches really hold up during the season they are applied!

rows of vegetables covered in white biodegradable mulch
A variety of vegetable crops grown on BDM in Gresczyk Farms in Litchfield, CT. Photo: Shuresh Ghimire
  1. How do yields on BDM compare to yields on polyethylene (PE) mulch materials? list of USDA National Organic Program Rules

Our team also looked at that over several years and found relatively similar yield results between BDM and PE mulch. Unfortunately, it’s difficult to state this definitively because in a survey of comparative studies, Martín-Closas et al. (2017) found better performance, worse performance, and equal performance between BDM and PE. But these were in a wide range of soil and climate conditions. The chances of BDM performing better than PE in an exceptionally wet summer in eastern North Carolina are slim- the stuff will break down more rapidly, so you do take something of chance there.

  1. Will last year’s mulch bits disappear by the time I am prepping beds this coming year?

Visual assessments of macroscopic BDM fragments (>2.36 mm) show that after 4 years of annual BDM application from 2015 to 2018 in northwest Washington, mulch recovery from soil in spring 2019 ranged from 23 to 64% of the amount applied (area basis), indicating there was no accumulation of mulch fragments in the soil even after repeated applications (Ghimire et al., 2020). Recovery further decreased to 4-16% (mass basis) 2 years after the final mulch incorporation in fall 2020 (Griffin-LaHue et al., 2022). Only paper BDMs (e.g. Weed Guard Plus) show 100% biodegradation within the timespan of this study, but the conclusion from the study was that BDMs are degrading and do not accumulate in soil after repeated use. The longevity is strongly influenced by soil texture [see footnote], climatic factors (annual rainfall, annual average air temperatures), and product formulations. As you might expect, biodegradation is more rapid in warmer, wetter climates and in medium textured soils which have the benefits of good water-holding capacity as well as decent porosity.

Yes, there is differential breakdown among the products, and some of it remains for up to a few years. Products that perform the best within a season also probably leaves residues for longer. These are often thicker. But that doesn’t mean it leaves behind microplastics. The limits of instrumentation make it nearly impossible to document the final-most end-stage of decomposition of soil biodegradable plastic, whereas microplastics are actually detectable.

  1. Isn’t BDM more expensive than PE mulch?

Yes, it really is more expensive to purchase the material- two to three times more, depending on thickness. Plan to lay out more cash on the front end of the season. BUT: don’t forget to calculate how much you can save on the back end. There’s an easy to use calculator available for download here. [Note: this link will take you to an Excel spreadsheet on your browser tab. You can download it by clicking on the “File” menu in the upper left-hand corner of the spread sheet and then click on “download.” After doing that, make sure that you enable the macro features.]

Below is what the calculator generated based on using 3- three-thousand-foot rolls of mulch on an acre. There are many assumptions built into this output, and the spreadsheet allows you to adjust all of those. But going with what they filled in for default estimates (labor rates, tractor time, dumping fees, and more), take a look.Per Acre Cost Comparison chart

In this case with all of the default values they started with (again, which you can adjust), it costs 11% more per acre. In case you’re wondering, many of the dollar values they used are on the generous side, so this may be an over-estimate. Over-estimating is always good in enterprise planning, at least to a point.

Other factors not accounted for in the spreadsheet:

  1. Aggravation/Procrastination factor: there’s nothing like an un-fun job to persuade you to do other things that seem less aggravating. But pulling out the drip tape is the only thing stopping you from disking over your plastic-mulched beds.
  2. Interference with timeliness of cover crop planting: getting those beds cleaned up speeds you towards getting your cover crop seed planted. Getting deep into the financial analysis, it’s entirely possible that the earlier cover crop planting recovers more nutrients and increases organic matter in your soil, which are things you can take to the bank.
  3. If you grow into the fall and winter, you have tunnels to clean up but also fall planting deadlines to keep. Deadlines like that can easily cause a delay of field cleanup because every day of waning daylight in mid to late September reduces fall tunnel crop growth.
  4. You are putting less polyethylene mulch onto the land and eventually, into the ocean. Hard to calculate costs associated with that, but they are real.
  5. Can it be applied in the field just like PE mulch?

Yes, more or less BUT three caveats:

  1. It is more delicate than PE so it has to be handled a little more gently.
  2. If it does get damaged while laying it down, decomposition will be accelerated
  3. MOST IMPORTANT- it should not be applied as tightly as PE mulch because it continues to tighten as the weather warms. If it is installed too tightly at first, it will split as it tightens up, and this will allow early summer weeds to take over.
  4. What are the common experiences of the growers using BDM in Connecticut?
  • PE mulch leaves more fragments in the field than BDM
  • The purchasing cost of BDM is greater than PE, so BDM appears to be expensive in the beginning of the growing season, but overall BDM is cheaper after accounting for disposal costs
  • Growers can prepare the field for cover crops at the end of the season when the crop is grown with BDM; the mulch is disked/harrowed in after the drip tape is removed, which does not require much extra field work. But in years with wet Fall, cover crops are delayed or cannot be planted when PE mulch is used
  • Even with mulch deterioration in the later season, no/minimal weed growth occurs
  • Some growers shared experience of mulch adhesion with cantaloupe, but has not affected marketability of crops
  • Removal of PE mulch and picking up fragments at the end of the season is the least liked job of growers
  • Weed control and yield is comparable between BDM and PE mulch
  • They do not have any concern with BDM fragments after incorporation in the field as their observation is that BDM degrades in a couple of years
pepper plants growing in rows covered by white biodegradable mulch
Pepper grown on BDM in Cold Spring Brook Farm, Berlin, Connecticut. Photo: Shuresh Ghimire

Think it over- it could change things for your late summer-into-fall transition… for the better. For a video testimonial from a Connecticut farmer, watch this video.

You still have questions about BDM, this FAQ FAQs about BDM might help.

Footnote: We did not specifically look at the relationship between soil texture and mulch biodegradation rate. However, a study from Brazil (Duarte et al. 2019) reports that the CO2 production was much higher when a biodegradable mulch was tested in sandy-loam textured soil compared to clay and sand-textured soil. In general, degradation rate would be higher in the soil where there are greater populations of microbial communities, the soil is not too dry or not too wet (balance of water and air in the soil pore/capillaries). This probably means greater degradation rate in sandy loam or silty loam soil than clay or sands.

Useful links

Sustainable & Climate Smart Agriculture

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CSQ4cxzWcAAJsboThe White House recognized UConn Extension educator Jiff Martin as a Champion of Change for Sustainable and Climate Smart Agriculture in October. The program features individuals who are doing extraordinary things to inspire and empower members of their communities, Martin was one of 12 individuals from across the country selected for this honor.

Jiff reflected on her role in sustainable and climate smart agriculture:

“When I came to Connecticut 13 years ago, I advocated on behalf of environmental leaders and farmers in Connecticut whose common cause was the preservation of working farmland.  This coalition, Working Lands Alliance, taught me the vital link between our quality of life and the land that sustains us.

Now I work with Cooperative Extension, connecting people to the farmland, farmers and flavors of local food. I am just one of so many CSQuJt0WcAAV4Cgchampions for change in our food system. Through our new farmer trainings, I have met bright, new farmers that are driven to feed their communities while sharing the realities of small-scale farming. I respect our partner organizations that use innovative food projects to improve their communities’ health and equity. And I am full of hope, thanks to the young emerging leaders that I meet through our AmeriCorps programs, FoodCorps CT, and Connecticut Food Justice VISTA Project.

In New England we have the capacity to produce and consume more of our own food through locally owned farming, fishing, and food enterprises. I see hundreds of endeavors across the region, from school gardens to incubator farms to food hubs to mobile markets. Without a doubt, our food system transformation is well underway.”

Jiff Martin is an Associate Extension Educator in Sustainable Food Systems, at University of Connecticut’s College of Agriculture, Health and Natural Resources.

When to Turn Under Spring Cover Crops?

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spring maple buds

I heard the peepers last night for the first time this year. There have already been a couple of sunny, almost warm, spring-like weeks in my neighborhood. Recently the overwintered rye has switched its dull reddish-green color scheme to bright green.

I remember reading a couple of years ago that stands of overwintered rye, if killed early, will provide the most nitrogen value.  Incorporating the young plants when they are just beginning to grow, will give as much as a 30-50 lb N credit. Providing soils are dry enough to drive onto, the best time to kill winter rye is when plants are no more than 6-8 inches tall and shortly after they have greened up.

Usually a light harrowing can kill winter rye when root crowns are small and the young stalks are not yet fibrous.  Allowing rye to continue to grow will put on biomass, however, the early spring nitrogen credit will be lost.  Nitrogen scavenged the previous fall and held in its roots throughout the winter will be utilized to put on rapid spring growth. Additional nitrogen will be required to mineralize it, when incorporated into the soil later in the spring.

If significant biomass is one’s goal, as well as field grown nitrogen, it’s better to seed a legume into one’s fall planted winter rye. Let the green manure cover crops grow to full maturity late into May or early June. Then turn them under and allow them to slowly break down to feed later summer cash crops.

Meanwhile, utilize the extensive root growth of an overwintered cover crop and benefit from the value of its winter carry over of “free” nitrogen. Many overwintering cover crops give the most value if you turn them under quite early.

— Eero Ruuttila,

Sustainable Agriculture Specialist – Scaling Up Program

UConn Extension – Tolland County

Information on early killing of spring cover crops came from the March 2011 Cornell VegEdge newsletter, authored by Thomas Bjorkman

4-H Education Center at Auerfarm

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The 124-acre 4-H Education Center at Auerfarm is located in the northwest section of Bloomfield, Connecticut.  Hartford entrepreneur and retailer Beatrice Auerbach deeded the farm to the CT 4-H Development Fund in 1976. Founded in the early years of the twentieth century, Auerfarm had been honored many times as a model site that included 60 purebred Guernsey cows, 20,000 chicken and 300 apple trees.  Today, the Auerbach legacy to the 4-H Education Center is expressed through the variety of 4-H education programs offered to families and children in the areas of gardening, agriculture and environmental science. Over 15,000 students and family members participate in year round 4-H curriculum based school science programs, animal clubs, and Junior Master Gardening activities.

auer farm

Visitors are invited to walk the property, go to the animal barn, the blueberry and raspberry beds and tour the newly established herb beds and the organic Master Gardener/Foodshare garden located on the hill above the animal barn.  The children’s herb garden, “Thyme in Auer Garden,” developed in 2012, provides a new area of horticultural discovery as butterflies, birds and flowers present themselves in a symmetrical raised bed. Children readily access and experience the colors, smell, and taste of the New England hardy perennials while they learn that plants provide medicine, flavorings, aroma and seed for wildlife.  With funding from a generous friend of the 4-H Education Center at Auerfarm and the hard work of Master Gardeners and volunteers, the herb garden has become a place to pause and reflect on the beautiful surrounding landscape.

The Master Gardener/Foodshare garden is a quarter acre vegetable garden used by Master Gardeners and 4-Hers as a demonstration site for learning the basics of environmentally responsible vegetable and flower production.  Students study growing conditions through understanding soil, water, insect, and disease management. The garden offers multiple opportunities helping in the seasonal progression of growing plants as well as observation of wildlife, especially birds.   Approximately 300 volunteers come out to help the Master Gardeners on the weekends.  Enjoying the farmland setting and giving back to the community provides a meaningful reward for the volunteers.  They do seeding, weeding and harvesting of approximately 3600 pounds of fresh produce for distribution to the community kitchens in and around Hartford.

The 4-H Education Center is open to the public daily throughout the calendar year.  You can see video of the gardens here.