Yes, you can plant those three plants in a 20” pot. I would not try to treat these plants as perennial, but rather annual, I.e., discard after they bolt, you may want to try to harvest the seeds for propagation. The celery may develop a bulb you might want to eat.

I do not know where you live, my comments are based on my experiences living in a “Mediterranean climate”.

Ditto what Jim said. One Plant may be more vigorous and want to take more space but all you do is pinch it so the other two have equal real estate. They’re cheap enough that I’d treat them as annuals and buy plants each year.

Both basil and coriander ARE annuals so hard not to treat them as such :-) And the parsley is only a biennial so the best you can expect out of that is 2 years

I keep forgetting that we must know where people garden. In Houston, coriander self seeds, so even if an annual, you have it forever (planted inground)

Thank you for the comments. I live in New Jersey, USA. I will start with a pot and harvest them regularly to contain growth. I will be happy if they last for one full season as most of you mentioned these plants are annual/biennial (parsley).

Although it's possible, personally I would not grow them all in one 20" pot. The coriander needs repeat sowings all season because it bolts quickly. That would disturb the other two plants. The basil needs full sun and plenty of heat while the parsley can take cooler temperatures, more moisture and can cope with some shade. The parsley could remain green through winter if protected.

I agree with Floral. I would grow them in separate pots.

Your weather will get hot, and coriander will bolt (flower and go to seed) quickly. You may delay it by protecting it from the afternoon sun, or giving it dappled shade throughout the day, but it will happen anyway when summer starts.

Basil may also decline pretty fast -- the East Coast is now home to basil downy mildew. It's traveling up north and you will have it soon. The last three summers I lost all my sweet basil in mid-July. The leaves get grayish and the whole plant dies.

Parsley will live for another year, but the second year it looks and tastes very differently. It produces few leaves that taste a bit bitter, and uses its energy to produce a flower and seeds. I would not keep it, unless you like the flower.

Sorry, I don't mean to be a bearer of so many bad news! I would grow them in separate containers, keep them protected from the blazing sun, and don't let the potting mix dry out.