If you are sure they are earthworms then there is no problem with them. They are there eating organic matter. Probably the potting mix itself, maybe the organic fertilizer depending what it is.

They aren't particularly desirable in a container as they will speed up the structural collapse of the potting mix, but they aren't really a pest either.

How did they get in to the container? Hard to say. If you have a drain hole on the ground they could enter that way, but worm eggs are pretty tiny and will be in pretty much anything worms were in so they could have come in with the potting mix too.

Overall, not a big deal. Personally I would repot the tree in due season and put the potting mix with the worms in the garden. If you don't want another pot full of worms wash out the inside of the container well so no eggs remain behind. Rinse the roots of any potting mix as well.

I follow the same line of thinking as JaG, but I think there is ample reason for concern. Your "Writhing with purplish worms" immediately translated into collapsed soil in my mind. I see a picture of the worms, set on consuming everything organic in the container and reducing soil particle size to something so small the soil holds huge volumes of water and little air. Then, the worms die and offer themselves as fodder for the anaerobic bacteria growing as a result of the airless conditions.

Since the citrus will be very unhappy growing in soup, and your admitted recent entry into the gardening ranks makes it unlikely that you would consider bare-rooting and repotting the plant on your own, I think I might look for something in the way of a soil drench that will rid the soil of the pests (be sure it's safe if you intend to consume the fruits).

My own course would be to completely bare-root the tree and do a 10 minute (root) soak in a 10% household bleach solution before repotting into a new (or sterilize the existing) container. I think you'll be continually plagued with the problem unless A) Cultural conditions become so severe for the worms they are all killed, including eggs B) You use a soil drench C) You employ the bare-rooting and bleach option.

Al

I am willing to try a repotting, though I have a very small first-floor balcony so it would be tricky. Could you say more about a soil drench?

I am wondering why the worms have thrived in this pot and not others. I'm guessing they came in with the marigolds.

I think your guess is more or less correct since you garden on a balcony. The worms didn't come in via the ground, right? :)

Something that went into the container introduced the worms. May have been potting mix, plants or what have you.

I will say that I am fairly amazed the worm population in your container got to the point they reproduced. Your citing large and small sized as well as the 'writhing' population indicates they no only thrive, but reproduced. No easy feat. Many folks, myself included have attempted to raise various worm species under controlled conditions and not met with great success as each species tend to require fairly narrow environmental conditions to thrive and reproduce.

I am one of those who thinks organic principles are largely ineffective in a container due to environmental instability, but your case proves the exception to the rule. Keeping worms alive in a container is hard enough, but establishing a reproducing population? Darn near impossible yet you did it without trying. In that sense, stellar work! :) Could you say more about a soil drench?

It is as simple as watering the container with something that kills worms and their eggs while not killing any living plant you care about.

Bleach will work if it isn't too strong a dilution.

Since you indicate you are willing to try a repot, please go that way.

You need to remove the plants from the container, prune away excessive roots(if your plant appears root bound) and otherwise rinse them 100% clean (so no eggs remain).

Then you need to dump all the potting mix and wash the container with a mild (10%) bleach water solution to ensure no eggs survive. Then, refill with potting mix and replant the trees.

Again, the worms themselves aren't a serious pest. It is only that they eat organic matter reducing it's particle size to that of clay soil which equals plant death in a container. In a garden they are terrific. In a container not so much, but worms still rock! Save what you can for your or someone else's garden.

Yeah, I have to say I was amazed too because though I haven't been gardening long, I haven't seen anything like it in that time. They were definitely reproducing because there were fat ones as well as babies, and all quite vigorous. I wonder if they like the fertilizer I put on for the citrus. It's a specialty fertilizer for citrus trees.

Thanks for all the advice!

May I ask you for some information, gmosko?

What were the ingredients in or the brand of your potting mix and what is the name of the fertilizer you used?

Also where in the US to do live (I am not asking for a street address, your state and the portion of it you live in are more than enough).

The reason I am asking is I am trying to wrap my brain around how on earth a worm population could thrive in an outdoor container to the point of reproducing. It simply rocks my brain that it is even possible in the US, but clearly I have something to learn.

Clearly you are under no obligation to disclose any information you are not comfortable with.

As a fellow northern California gardener I am not surprised at worms in containers,a regular occurance for me. As my containers are on the ground the source of the worms is not a mystery. Al

I am in the SF Bay area.

The potting soil I used was organic store brand from Summer Winds Nursery. I can't remember the brand name, sorry, but I will look for it next time I go to the store. The fertilizer is E.B. Stone Organics for citrus trees. In short, nothing remarkable.