It's a water sprout; notice the large thorns. If it were mine, I would do 3 things; first I would prune it back by about half; second, I would move it out of that terracotta pot into plastic, ceramic, or some other non-porous material (evaporation thru the clay cools the soil and citrus roots do not like to grow into cool soil); third I would lose the stake, little by little if necessary.

Hi John,

Thanks for the info. I'm new to plant care and didn't even know about suckers or water sprouts.

When you say prune it by a half, you mean the sprout? Because I did some reading after seeing your comment and the advice is mostly "cut it at the base", but for established trees, not for little ones like this guy.

I'll put it in a ceramic pot as soon as I can.

When you say lose the stake little by little, does that mean I should make it shorter little by little or try to remove it completely, but experimenting with the time to do it?

Thanks a lot for the comments, I'll do just that. Keep you posted on the development in a couple of months.

After a month since I pruned the water sprout and removed the stake the lemon is showing new growth in a few places — one at the very top of the leader (which seems great) and a couple of them at random heights. Today's picture:

Worried that the lower two would be water sprouts.

If you want taller trunk base for your tree, you can remove the lowest of the new grown branches, it is too close to the soil.

Great advice, thank you. I removed it :)

Also, I browsed through your profile and noticed some images of citruses and avocados so I tried to ask a direct question, but turned out I posted a public question about an avocado tree, which you can see here: https://www.houzz.com/discussions/this-avocado-looks-really-healthy-what-did-you-do-to-achieve-it-dsvw-vd~4539913

It would be really helpful if you have some knowledge and experience with avocados.

Thanks :)

See your avocado post, the answer to your question is there ;)

[late-spring UPDATE]

So, I started fertilising the lemon tree two months ago and it has been receiving a lot of direct sun for a month or so already. The leader branch has put out about 6 new leaves on around 7" of new length. I've been out of the apartment for 6 days traveling and this is what happened:

As you can see on the right — a huge water sprout appeared! It has large thorns, a swirly branch and it grew out really fast.

I know I should probably prune it, but I'd like to hear it from someone else :)

What I'm thinking, actually, is why is that happening? Why are the water sprouts appearing. It seems the lemon is lacking something, or I might be wrong. The big one is not the only one. A couple of smaller ones started growing too:

What I'm actually most puzzled about is that the leader branch is not completely upright, it goes a couple degrees to the side (to the left on the next image). The other side being the first water sprout that I pruned in the post above.

Also, the one I think is the leader is showing potential new growth but nothing is happening for a while now.

Any advice is welcome — I will be brave and prune it wherever it's necessary :)

Ya know...those are just really healthy shoots. Water sprouts are not a bad thing. Just normal. I usually pinch off the thorns while they are in the soft stage. No big deal. You're just growing this as a foliage plant anyway (it's a seedling, right?), so trim it to any shape you want. But I'd let the vigorous shoots grow out a bit, then pinch off the last inch or two of them. Hopefully they will branch out even more and give you a fuller plant over time -- provided they are getting enough sunlight.