Retting Hemp Stalks – Do Your Trials This Year!

Harvest is almost here, and many farmers are thinking about how to capture value from their hemp stalks.  The truth is that plain baled stalks have little value.  In fact, if they are not properly stored, they will rot pretty quickly, but Retted stalks, properly stored, have the potential of commanding a price.  At the moment, there are not a lot of decortication options out there, and we are working on that. In the end, your stalks will be worth more retted than rotted. 

We ran several different cultivars through our prototype decorticator earlier this year, and the results varied widely.  What has become clear is that the success of a decorticator is largely dependent on the condition and cultivar of the feedstock.  We realized that we need to alter our design to get our customers the best chance at successful decortication, but we can get a much better separation if the stalks are are prepared by retting.

So how do you ret hemp stalks?

Retting (Dutch for rotting) is the breaking down of the natural glues that bond the outer bast fiber to the inner woody hurd.  Note that the glues which bind the fibers to each other are different than the fiber-to-hurd glues.  Retting is usually the degradation of the fiber-to-hurd glues so that the fiber and hurd can be separated.  Further treatment is required to separate the fiber bundles into finer fibers.  Classically, the stalks would be retted either in retting ponds or in their fields using natural moisture.  Pond retting will break down the glues relatively quickly, but it also degrades the hurd and, more importantly, makes the pond water a nasty waste product.  In times of old, the rivers of Europe suffered greatly from retting pond wastewater.  Until we are ready to invest land into retting ponds and wastewater evaporation ponds, large scale water retting is not feasible.  Research is being conducted into other controlled retting methods (enzymatic, chemical, microwave, etc.), but as of now, those are not readily available on a commercial scale. 

That leaves us with field retting (or dew retting, as it is often called).  For this method, the goal is to provide enough moisture to the cut and windrowed stalks for the glue eating microbes to do their work without getting the stalks so wet that they rot.  Think damp/dry cycles; enough to keep the microbes eating the glues but not enough to allow mold and rot to develop.  The glues will likely be reasonably degraded after 6-14 days.  Hopefully, the nighttime dew will provide enough moisture to keep the microbes active, but rains and swings in the humidity levels will all affect the amount of time required to break down the glue.  The glue will be consumed on the side of the stalk where the moisture has settled.  Watch your stalks as they ret and notice that the side exposed to the moisture (the top) will turn a different color.  The stalks need to be flipped (usually with a windrow merger) to expose the other sides to the moisture and allow retting.  The stalks may need to be turned 2 or 3 times, meaning retting may take up to 4 or 6 weeks.

How do you know when the retting is done?  Cut a short stalk piece 4-6 inches long and drop it into a jar with water in it.  Shake it vigorously.  The fiber should be separating from the stalk from the action of the water.

Once the stalks are retted, let them dry in the field and collect them either in full length bundles or bales.  Don’t store the bales on the ground; put down pallets or dunnage to maintain airflow under the bales.  Bales on the ground will rot.  Also, put more pallets or dunnage between the rows of bales allowing air to circulate between rows.  As the bales sit, exposed to natural humidity changes, they should continue to ret.  Cover the top of the bales with a tarp to prevent rain from saturating them, but leave the sides open.  Your bales will last longer in storage, giving the decortication people a chance to get their (our?) act together and get to your site.

As an alternative, stalks can ret standing.  Once the top is harvested for seed, grain, or flower, the stalks are left standing in the field to die and undergo the same wet/dry cycling.  We don’t know how long the retting will take and it will certainly vary with temperature, latitude, and myriad other factors.  In general, this style of retting will continue through the winter.  Once they are retted, then cut, bundle or bale, and decorticate.  This method has the benefits of keeping the stalks off the ground, providing a cover crop, and more even retting around the stalk.  However, many farmers we have spoken with do not want to leave the stalks occupying their fields for months.

This is certainly not an exact science, and results will vary with cultivars, temperatures, humidity levels, etc.  But if you are trying to gain value out of your stalks, that value can be increased and preserved if the stalks are properly retted and stored.  If possible, take the opportunity to experiment a bit and see how long, how many turns, how much humidity, etc. is required to ret your stalks.  Be ready when the decortication equipment is available in your area.  We would love to hear about your trials and results.

By the way, there is a segment of the industry that promotes green decortication.  We have not seen very effective automated separation of hurd from green fiber, and the fiber needs to be dried immediately to prevent mold and rot.  We believe that classically retted hemp stalks will produce higher quality products with better results from automated machinery, at least at this time.

For those specializing in fiber and not quality hurd products, a water ret will give better, more uniform color and happen faster.  Contact us for more details if you have a way to dispose of the wastewater and are dealing in relatively small batches.