Most of the weed seeds are tomatoes, with a few peppers thrown in here and there.Most of the weed seeds are tomatoes, with a few peppers thrown in here and there.
I almost didn't take photos because I thought this was so embarrassing, but eventually decided to do it for the good of the food growing community.I almost didn't take photos because I thought this was so embarrassing, but eventually decided to do it for the good of the food growing community.
Starting weed seeds. Unfortunately.Starting weed seeds. Unfortunately.
  • Most of the weed seeds are tomatoes, with a few peppers thrown in here and there.
  • I almost didn't take photos because I thought this was so embarrassing, but eventually decided to do it for the good of the food growing community.
  • Starting weed seeds. Unfortunately.

I recently read an article about a study that found that using worm castings in your seed-starting mix can help prevent damping off. I’ve never had a problem so far with damping off, but since I have a ready supply of worm castings at my disposal, I figured it was worth a shot.

Here’s what I’ve found so far:


  • Can prevent damping off
  • Provides nutrients to the seedlings


  • SEEDS. One of the unfortunate things about vermicomposting is that the worms don’t do anything to deactivate any seeds that you might put in your compost. All the seeds you put in the compost will return out of it, unchanged. This means that if you add the castings directly to your garden bed, you’re introducing “weed” seeds into you garden  (they may be tomatoes, but if they’re unwanted tomatoes, they’re weeds). And if you add the compost to your seed-starting mix, you’re introducing weed seeds into what you want to be a seed-free environment, except for the seeds you are actively planting. This is a problem.
  • Dries out more easily. I use a standard Jiffy-style organic peat-based seed-starting mix. The peat holds water really well, which is wonderful because I often forget to check on and water the seedlings. (Sidenote: I recently discovered capillary mats, and am excited to try out this lazy man’s efficient form of seedling irrigation.) Using the worm castings, though, the soil blocks have dried out much faster. This can be a problem with seed-starting, because you’re trying to provide the most favourable environment possible so the seedlings can get off to a strong, healthy start. I used about half a tray of worm castings to about ~15 quarts of seed-starting mix. I have no idea if this is a proper ratio. It’s possible that I used way too much, and that’s why the blocks dried out. Either way, though, this is a good thing to know: worm compost doesn’t hold water like peat does.

To me, the seeds that have popped up are the most challenging aspect of this. I can control the water, but I can’t control the germination of weed seeds. Plus, the weed seeds can potentially a) get in the way with or interfere with the germination of the seed you actually wanted to plant, b) when you pull the unwanted seedling out, there’s a possibility that you could damage the desired seedling and c) if you let the weed seeds get too big, they could potentially steal nutrients away from the seed you’re trying to grow.

I’m not anti-worm compost for starting seeds at this point, but I think I’ll need to be much more careful about what goes into our compost. I’ve tried to keep seeds out of the compost, but there have been a few moments of weakness in the middle of a canning session when I’ve dumped a bunch of tomato seeds into the bucket. Lesson learned.

In the future, if I want to use worm castings in my seed-starting mix, I think I’ll need to put the castings under lights for a few days to let any existing seeds germinate. At that point they will be easy to remove, and then I can add the (mostly) weed-free compost to the rest of my seed-starting mix.

I was really excited about the worm compost study, but as it turns out, it’s slightly more complicated than you would hope. Oh, science.

18 Responses to “The pros and cons of starting seeds with worm castings (vermicompost)”

  1. Sharon Downs
    18 February 2012 at 5:58 pm #

    I also had a problem with unwanted seeds in my worm castings. I’ve read that worms digest 100% of what they eat. That’s not true. Last year when I added my worm castings to my seed starting mixture, I got so many volunteer seedlings that came from the kitchen scraps I fed them I couldn’t find the seed I intentionally planted. I’ve endlessly looked on the internet how to right this problem but nobody except this last poster addressed this pressing issue. I need help. Do I just stop feeding them kitchen scraps? I’ve tried to be mindful as to not add seeds to my food scraps but that is hard to monitor.

  2. CoMo Homestead
    19 February 2012 at 1:38 pm #

    You’re right, worms don’t eat everything they’re given. They’re not big fans of citrus, for example. They definitely don’t eat or digest seeds. I have a hard time imagining worms eating even small seeds (especially since they only eat things which are decaying, and seeds are obviously not), never mind the bigger ones, so I think it’s more likely an issue of them just leaving the seeds where they lie, rather than not digesting them.

    I don’t know that there is a good way to get around this. You could put the worm castings in your compost and let the heat of the compost kill the seeds, but that seems to defeat the purpose of vermicomposting. The simplest solution I can figure out is to just not put seeds in your worm bin. The vast majority of the volunteer seeds in the castings I used were tomatoes, so now I know not to put the tomatoes in there. They can go out to the regular compost, but not to the worms.

  3. Faye
    5 March 2012 at 11:44 am #

    Could you put some of the worm casting in a container an heat them in the over @250 for 20 mins? or would that damage the castings for fertilizer

  4. CoMo Homestead
    5 March 2012 at 12:13 pm #

    Well, the main purpose of using the worm castings (especially for protection from damping off) is that they are full of beneficial microbes. The oven would kill them off, which then defeats your purpose of using castings to begin with.

  5. Christine
    9 March 2012 at 2:03 pm #

    I like the idea of putting the castings under light to germinate the seeds that may exist prior to using it to start wanted seeds. If you pulled the seedlings out regularly hopefully the seedlings wouldn’t leech the nutrients out of the compost.. After the next time I harvest I will definitely try to avoid putting seeds in my bin, I have tomato plants growing in my rosemary.

  6. CoMo Homestead
    9 March 2012 at 2:36 pm #

    No, the seedlings shouldn’t pull much (if any) of the nutrients out of the castings when they’re that small. If you end up doing this, let me know how it goes!

  7. Sasha
    24 November 2012 at 4:32 pm #

    I love this subject. I have been using worm castings in my soil mix when sowing seeds in trays. So far I have had 100% germination of tomatoes and lettuce. That is good enough for me! Also haven’t noticed any damping off and the compost in the trays is actually staying pretty moist, which I note is the opposite of your experience where the soil has dried out too quickly.

  8. CoMo Homestead
    28 November 2012 at 10:06 am #

    Interesting! Do you know what proportions of castings (50/50, 25/75, etc.) you’re using?

  9. Galleon
    24 January 2013 at 4:43 pm #

    Here is an idea for those interested in adding worm castings to your seedlings…..get weed free castings :)

  10. The Garden Patch
    4 March 2013 at 10:28 pm #

    We have a couple of Greenhouses and raise Veg. plants and flowers. We also raise worms for the casting. you are correct about the seeds in the casting but if you feed your worms seed free food then you want have that problem. we feed ours a seed free diet so we are able to use the casting with out the weed problem. we sell our casting for $15.00 a five gallon bucket plus shipping.

  11. Stephen
    17 March 2013 at 5:45 pm #

    Would the problem with seeds in castings remain when using a multi-bin system wherein the top bin has compost/veggies and the next bin down has castings? It would seem that if the worms do not eat seeds then most of them would likely stay in the top bin. Does that sound right?

  12. Ives Organics
    6 April 2013 at 4:55 pm #

    I have some beautifully sifted castings intended to give my seedlings a nutrient boost. I plan on spreading the castings out under a light in a mini greenhouse with a germination mat underneath, give it a week and pull out any unwanted plants. I will let you all know how it goes.


  13. Nirvana
    27 January 2014 at 5:43 am #

    I normally freeze my vegetable scraps, defrost and then feed to my worms. They process the scraps faster and I don’t have too much trouble with growth from seeds

  14. Gerry
    5 February 2014 at 4:46 pm #

    I make a soil mix using one part worm castings ,one half part play sand and two and a half part peat moss or compost with great success . If you are worried about seeds then maybe make worm tea , then add it to your soil mix ,it seems to be a better product anyways .
    Hope this helps

  15. rowin
    26 February 2014 at 8:11 pm #

    just read your post, and enjoyed it very much.
    I came upon it, because a neighbour of mine ended up with seeds germing inside of the wormbin !

    I have read that the Bokashi bin works good, when you want to be sure the seeds won’t germinate. The die in the bucket.
    After fermenting the food waste with the seeds, it can still be added to the wormbin, to get the plus of the vermicomposting system

  16. John
    16 March 2014 at 7:12 am #

    I read somewhere that about 20% castings is right and to
    put them in the seedling plug before the starter mix – I.e. as layer on bottom. Reason given was weed seeds will be choked out and
    castings with all their nutrient goodness will be there for the seedling when the roots grow down and will need it.

  17. Vidyut
    11 July 2014 at 4:50 am #

    Would it be useful to vermicompost compost and use that for the seedlings? That way, any seeds will have died in the heat of the compost, and the vermicompost will still be useful.

    The other idea that comes to mind is to create a separate vermicompost bin which can be smaller (easier to feed – less effort spent on controlling) that contains carefully seed free materials, while the regular bin can take everything else. Seedlings will hardly need large quantities. A small bin ought to work for most people.

  18. Nick
    4 November 2014 at 4:45 pm #

    I’ve seen a couple of comments that were getting pretty close to the ideas I have on how to avoid seeds that might germinate in worm castings. Please know, there’s no reason for me to believe that either of my ideas might work–but for an adventurous soul, they might work.

    Heat kills organic material–why not boil worm food before feeding the worms? I think that should kill seeds; seems like it would also make the food nice and soft for the worms, though only after sacrificing some of the nutrition that stays in the boiled water. The micro-organisms in the castings are products of the worms’ digestive system, so lost nutrition from the food might not matter–might. I invite other thoughts around lost nutrition from boiling, it’s something I’ve thought about doing in the past but never have.

    The other idea I have is to submerge worm food in water and freeze it. I don’t know the science behind it, but seeds have to be dry if they are going to be preserved in a dormant and frozen state. Completely saturated seeds that freeze should die, and then they should rot in the worm bin, and then the worms should eat them.

    Good luck!