What to do if too many seedlings are in the same pot?

Question

I have attempted to grow cacti from seeds I collected from my own cactus. Since the last time I tried this (last year), only about 5 seeds started germination and then quickly died, this time I thought I'd simply use some more seeds to increase the chance that any at all will survive.

Now unfortunately (well, actually that's a good thing, I guess) a lot of my seeds started germination so now my pots are pretty full with small cactus seedlings:

While in general it's great that so many even survived, I'm concerned that they won't have enough space (especially those in the lower pots in the image) and it will negatively affect their further growth.

Since they're so young (only a few weeks old, an individual seedling is maybe some 1-2mm), I assume I cannot transplant them without a large number of those transplanted dying. So my options are

1. Leave it as it is and hope it regulates itself, then transplant in a few months (?) when they're larger

2. manually remove some seedlings to give the rest more space

I'm not sure which of those options is better since I don't have much experience growing plants. The problem with 1. seems to be that by leaving all seedlings in one pot, maybe all will have problems growing and all will die (instead of just the weaker ones). However with removing cacti (and possibly trying to transplant them), I might damage the seedlings right next to the one I remove.

So what should I do? Is there any other option than the ones I've listed? If it's relevant, those are Mammillaria cacti (I'm almost completely sure that the top row are Mammillaria Spinosissima Pilcayensis and the rest are Mammillaria Mystax - I could provide pictures if needed).

TL;DR: If too many cactus seedlings are in the same pot, should I let them be, remove some or do something completely different?

---

The most similar question to mine I could find on this site is Are there too many Asiatic lilies in this pot? but I don't think it's a duplicate since seedlings might need different treatment as they're more vulnerable (hence the transplantation problem).

Answer 1

You seem to have more than a hundred seedlings overall. Are you willing to grow every single one of them into adult plants? I'm assuming not. If that's the case, you'll be discarding them at a later stage anyways.

So, let's explore your options:

• If you leave them be (option 1), some of them will of course die from competition. But the important thing is, when you'll want to transplant them, you're going to have to fight a mess of intertwined cacti. Many of them will look irregular, stretched or deformed from growing in a packed environment, just like the children around an adult one. You'll tear a lot of roots apart. You'll have a hard time separating entangled spines. You'll need to use your bare hands for the delicate work and that means some stinging will be inevitable. All in all, after all that work and suffering, you'll still lose some of them, maybe even precisely the ones you meant to keep.

• If you discard some seedlings now (option 2), you're blindly selecting which ones you'll grow. In order to give them enough room, I'd say you'll need to remove at least 80% of them. So, there's a good chance that you might lose the best performers. You might end up keeping some of the weakest, fussiest ones, which will probably die shortly after anyways. There is no way to tell right now.

So, what if you just transplant most of them, enough to make room? Of course you'll lose some seedlings, but you won't lose anywhere near 100% of them, so it's still better than discarding them right away. With the kind of plants that produce lots of tiny seeds, this is the absolute way to go, you need to seed way more than you expect to grow because most seedlings will just die.

At this point, they are so small that you'll be able to take a whole seedling with its root and a bit of soil attached without breaking anything. You're not likely to damage the ones you don't transplant, except maybe for the bottom right pot which looks really crowded. Just try it: pick one up gently with some tweezers, or even a toothpick, and drop it on the surface of another pot with moist soil. At this stage you don't even need to "plant" them right, because the root will follow the humidity of the soil even if you put them upside down (of course it won't hurt either).

Doing this you'll achieve three things:

• Both the seedlings you transplant and the ones you don't will now have enough room to grow.
• You'll let all (most) of them to grow for a bit longer, so you can see which ones perform best and select those for growing.
• You'll put some stress on them, so the weakest ones will die sooner. This is actually a good thing, because you don't want to waste your time caring for some ill-fated seedlings when you have plenty of healthy ones.

Answer 2

I basically second willyjoker's answer(+1).

The three options:

• Leave them be. None will likely develop into a mature plant.
• Replant. Some will die, some will survive. Some people advise to wait for the first true leaves(do cacti have leaves?), some suggest the root is so small now that it's cool.
• Weed them off. Better than the first alternative.

In any case I would suggest leaving 2 plants per pot in case one of them dies.

Answer 3

Thinning seedlings is standard practice, as is planting too many seeds either because they are tiny and hard to handle or because you're not sure of the germination rate. If the seedlings are very small and delicate (as those in your photograph), I recommend using a narrow-point pair of scissors to snip off some of them without touching/disturbing the others. Just cut some of them and in a few days or a week you can snip some more. Always thin (cut) the weaker seedlings and leave the stronger ones.

Answer 4

Replanting them all is a great idea, but usually if I just need to thin I prefer to cut (or pinch) them off, in order to avoid disturbing the roots of those I am leaving.

Answer 5

I've never grown cacti, but in general, transplanting seedlings once there is a pair of true leaves is helpful, in preventing damp-off and legginess.

You can tease the seedlings apart gently with a pencil. But water well right before starting.

Another thing that will help prevent damp-off: don't over-water.