Gas Powered Shredder / Wildflower Seed Thresher




About: Dad and hubby, good food enthusiast, solar energy, boating, making stuff, melting stuff, and raising chickens.

I made this for breaking up seed heads for prairie restoration, but it ended up being an good leaf shredder too. I made an electric one earlier, but this gas powered one works so much better because of the lower RPMs and higher torque. This is very fun to use, the neighbor kids raked my whole yard just so they could throw leaves into this thing.
When collecting wildflower seeds for prairie restoration, hundreds of seeds can be enclosed in a seed head, (think of sunflowers.) I needed a way to quickly and cheaply break down the heads to extract the seeds for broadcast planting. Sure I could pay someone to process the seed or I could buy the expensive equipment, but this seems to work well and cost me about $5. I picked up almost everything from curbside trash at spring cleanup. The $5 went to new fuel line to fix the junked weedeater and to some .25" hardware cloth.

This doesn't work well with hard seed heads like echinacea, penstemon, and rose hips. For those seeds I made a smaller version of this using a five gallon bucket and a blender motor. That thing really rips things apart well without damaging the actual seeds.

You'll need:
A round 30 gallon trash can, already had this, new $20 (Use a plastic can, this metal one cuts the line a lot where the screen is attached.)
Gas power string trimmer, free or $60 new.
Lawn mower handle, free from junked mower.
.25" hardware cloth.
Some scrap wood, 1x6 board, 1x2 and 2x2 pieces.
Curtain rods or plumbing strap.
Deck screws and some bolts.
A throttle adjustment lever from a mower.

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Step 1: Cut the Trash Can and Add Mesh.

Cut three or four sections out of the bottom of the can with a drill and some tin snips. Make it look like a wheel with 2" wide mags. Cut the mesh/hardware cloth to fit the can. Use some 1x2 pieces of wood to reinforce the mags and hold on the mesh. Drill screws into the wood from the inside of the can. Snap of the screw tips with pliers if they're too long.


Step 2: Make Legs and Motor Mount.

I used an old lawn mower handle to make the legs. Take the handle off the mower deck and take the two pieces apart. Use a pipe wrench to twist the handles so the flat tips match up with the can as shown in the picture. You may have to flatten the tips with a hammer and drill your own holes in the handle depending on the mower manufacturer. Drill through the can and put the bolts through the holes from the inside so the nut is out.

Attach a board to the bottom of the legs to mount the motor.
Drill holes in the bottom of the legs to attach the 1x6 or 1x8 board with screws. The length of the board depends on how much you shorten the weed trimmer shaft, This one is 2.5' long.

At this point you may want to attach small stabilizer 1x2 boards perpendicular to the motor mount board.

Step 3: Prepare the Gas String Trimmer.

Remove the shaft from the trimmer (save all the parts.)

Bend a 90 degree in the shaft with a tubing bender.

Remove the drive cable and shorten the shaft with a hacksaw; I removed about 12" for this project. (Remember how much drive cable sticks out the end before the cut.)

Cut the drive cable with some bolt cutters.

Now you need to make a new tip for the drive cable with a grinder or file; just grind the fresh cut tip to the right shape. I think generally they're square tipped.

Step 4: Mount the Motor and Go.

Before attaching the shaft to the motor, slide the loop handle grip back onto the shaft and use it to prop up the trimmer head into the can; look at the picture to understand this better.

Center the trimmer head in the trash can and adjust it so the string spins .5" to 1" above the bottom of the can. Use something to screw down the handle to the board once you have everything lined up; I used half of an electrical fitting clamp.

Use some wood to block up the engine like in the picture. Then use plumbing strap or whatever else you have (I used flattened curtain rods) to brace the motor to the board.

You can mount the hand trigger throttle assembly to a lawnmower throttle adjustment lever so the rpm's can be set to be constant; thus freeing hands for loading seedheads.

Step 5: Video, Tips, and Extra Pics.

Turn it off before reaching inside, durr.
Wear eye protection.
Wear respiratory protection if you're sensitive to allergens.
Clean the engine air filter often because this makes a lot of small debris.
Use a piece of wood to tap the button on the trimmer head to let out more string when needed.
Don't forget to lay out a tarp to catch your seeds.

I considered adding wheels, but this is pretty light and easy to carry.

A patched together video, some sound is missing because of the old camera.

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    34 Discussions

    If you are lucky you survive the resulting injuries.

    A weed whacker spins around 10-12k rpm.

    A table saw spins around 4k, and has a propensity to kick back with gruesome results, if you triple the speed and replace the solid stabilizing frame with a lightweight pole meant to control about 6 inches of spinning fishing line, you have almost certain loss of life or limb


    6 years ago on Introduction

    Now I'm thinking I have a dead mower with 5/8 shaft and blade flange. I also have a 2 horse sealed electric motor with, yes, a 5/8 shaft. I see a visit to the emergency room in my future.

    I think as long as the blade direction is correct and I cut and balance it with enough clearance for the can I can make it work. Might be easier if I had a reversible motor.


    6 years ago on Introduction

    Very nice ible! I have been looking for a low-cost solution to getting prepared mulch for my raised beds. I plan to mod it a little - build the wood platform base independent of the trash can, could even be a square with HW cloth mounted under retainer strips - mount the trimmer head underneath, then place a bottomless (cut out) plastic trash can over it all, fastened on with 4 angle braces (thin plywood shims inside the plastic can for strength. Or use luggage snap locks for quick release to service/change trimmer string. Thanks - I know this is an old ible, but I just found it looking for a way to get a mulcher here in Costa Rica - the ones available here cost over $1k!


    9 years ago on Introduction

    very interesting project. Quilter2 was asking about an 'industrial' version. My thought on this would be to try using a large electric motor like those found in a dryer. I would mount it off to the side and use a belt and pulley to turn a driveshaft from an old lawnmower. You could then put a mower blade or two inside the can. I think I have just found my next project.:)


    9 years ago on Introduction

    Is there a way to work with alpaca droppings? When fresh they can have a lot of moisture, but I could handle them to just drop the individual "beans".There is stings of grass. Nothing at all heavy. What about wet Big Leaf Maple leaves that have clumped together? I really need the INDUSTRIAL version. But would be so willing to add handfuls at a time ,not shovelfuls Please can someone make this happen? So respectful, quilter2


    10 years ago on Introduction

    I saw your piece in farm show. And for like 5 minutes I was having all these wierd deja vu moments trying to remember where I had seen it before. Then I actually read the article... Good stuff.

    2 replies

    Reply 10 years ago on Introduction

    Thanks, I love that magazine and was happy to have two features in it; this thresher and my pipe shovel. If there are other Farm Show fans here, maybe someone could start a group.


    Reply 10 years ago on Introduction

    I saw both, but I had no idea it was the same person behind the two. I don't look too deeply into things...


    11 years ago on Introduction

    Just started making my own compost and found out i could use an electric strimmer to break down the leaves the plastic line kept breaking on the small sticks though :S I think it might have broken hitting the sides too. This is a much better idea will have to make, thanks!

    6 replies

    Reply 11 years ago on Introduction

    Small sticks are sometimes a problem with this machine when they build up in the can. One way to keep the string from breaking a lot is running at a lower RPM and/or use higher quality string. If I wanted to spend money I would add an aftermarket head with the tough "strings/cords"

    Big Bwanarobbtoberfest

    Reply 11 years ago on Introduction

    Or you could use a light chain (( not 60,000 lbs logging chain )) it flexes well, It has enough weight to break heavy sticks with ease, and takes what ever might other wise damage a string or wire.. And doesn't need much balancing just make sure it centered or if your using two peices make sure they are the same length..

    thygreatoneBig Bwana

    Reply 10 years ago on Introduction

    Thanks for clarifying about the logging chain - I'd hate to have tried it and then wound up with a big piece of chain lodged somewhere in my body. But seriously, I think the light chain idea is good, but you would want to make sure it was very secure.


    Reply 11 years ago on Introduction

    Solution: replace the plastic line with coat-hanger wire as a simle loop around the spool....This will *eventually* break, but more likely will break far less often. Ideal if you counterbalance with a symmetrical line on the other side too. Run at full speed for added brutality points.


    I wouldn't recommend copper wire for this, as it would fatigue rather quickly. Another alternative is to use bicycle spokes as they are quite strong and meant to take a beating. Their shape and consistent lengths should make it easier to balance the rig. Simply drill two holes opposite and run them through with a washer for support. If you need shorter than you have, just cut them both to the exact same length. I did this with a few other trimmers (bending the ends over in a small loop for safety), and they have made up for over 4 spools of the best trimmer line money can buy so far. Just remember to bend the loops away from the direction of rotation so they don't snag material Don't worry about spokes breaking under centrifugal force, you would need to exceed at least 10,000 rpm or so for 12-inch long spokes before that was a concern. Just food for thought...