Lemon balm eventualy gets tall and woody, although you could keep it clipped.

My personal (visual) preference would be two or three thymes, with at least one being silver thyme. I just think that would make a nice, frothy base for the tree, and if you are able to keep all outside they might perennialize.

Don't put pineapple sage in there--it would take over the whole pot, as would most mints. Oregano might work, though, if you like that.

If you like onion, you could also put in some chives, which eventually have edible flowers.

How wide is the pot?

The pot is about 19" wide and 16" tall. (50cm W, 40cm H)

Chives don't do well with heat - I don't know what your climate is like. Chives don't like the most southern reaches of the United States. If you are that warm or warmer, I would not try them.

I wouldn't personally do woody plants. They have deeper and woody roots. Would make for a tangled mess come transplant time.

I wouldn't personally underplant the lemon. But if you decide that is the way to go, I'd probably stick with something herbaceous, something with more shallow roots. Something temporary. Something that would trail over the pot rather than grow upwards. Maybe petunias, tradescantias, and so on.

FataMorgana

Nasturtiums?

Flowers, edible, trailing.

I absolutely wouldn't put anything else in the pot. It's not that big and the lemon needs all the nutrition it can get. I have a Meyer in a similar pot and it really can't be doing with any competition. That pot is definitely not any bigger than the lemon needs. Even without competition it will need regular refreshing with new compost at the top and also feeding with a Citrus food.

I have lemon balm under one of my meyer lemons

Don't put anything else in the pot, which is too small for a lemon tree anyway. Citrus do not like competition around their roots.

I have a potted Meyer Lemon (Central Victoria) and am underplanting with dwarf snow peas for Nitrogen fixing in the soil. They arent a long term crop so wont compete with the lemon while its growing.

`i agree that your pot isn't all that big for a Meyer Lemon. I wouldn't underplant it either.

I just bought two and have the same question. I thought I might put a rosemary, basil, oregano under them.

I underplanted several annuals with one of my Meyers and noticed it negatively impact the tree compared to the Meyer that I did not under plant. I removed the annuals and fertilized. Now I wait.