Without more info, I'd have to go with the odds favorite - too heavy a hand by the watering committee. Herbs like a fast soil & to be kept on the dry side in full sun.

Al

It is endlessly searching for the nutrients it loves (which is why it will take over the yard.) Repot in a bigger pot, or divide it and give it more soil in the new pots, or top dress it with fresh soil if there is room (just put it in over the stems.) You may also have something eating it all.
The additional soil may also help it keep moisture. I find my mints suck up all coffee grinds and other stuff I can give them. Mints are hard to kill but they can also live on as ugly zombies for a long long time if you don't feed them right.

Mint like other herbs will flower and go to seed eventually. To prevent this, you can "harvest" the mint (ie., cut back and use the snippings) and the rest of your herbs to keep them bushy and extend their lifespan by delaying their flowering. In the case of mint, there are many ideas for storing including freezing in ice cubes, etc.

Cilantro happens to be a short lived annual herb that dies after flowering and going to seed.

Ankrara's Hobby Corner

Coriander (or cilantro) and dill happen to be biennial herbs of the family that includes parsley. Thought you might like to know. ;o)

Al

My coriander always flowers & sets seed extremely fast (~6 weeks following germination in cool weather, 3-4 weeks in warm weather). I've seen it referred to as both annual & biennial. That makes me wonder - are there two distinctly different types of coriander?

Hmmmm. After I read your post I did some looking around. It is referred to as biennial in most, but not all more technical sites, and as annual in more sites than as biennial. It appears that either label shouldn't be questioned. Pardons begged. ;o)

Though where a plant is grown cannot make a difference in whether it is an annual or a perennial (e.g. even though an impatiens freezes in MI, it's still a perennial), it may make a difference in determining if it is a biennial or annual. IOW, if coriander is grown in conditions favorable for flowering & setting seed, perhaps it can complete a life cycle in one year. On the other hand, if conditions are less favorable, slowing development, it may take two years for the plant to complete a cycle. In this case, or under these varied circumstances, the same plant could fit the technical definitions of both annual and perennial.

Nothing scientific above, just me musing aloud.

Al

Thanks Al. I did some more looking around. It seems there are two types of coriander - leaf coriander & seed coriander based on the amount of foliage produced and the time taken to go to seed. Is it possible that they are, respectively, biennial & annual?

Cilantro will go to seed (in my area, anyway) in the spring, even if you're doing everything "right." That's ok--leave the white flowers alone and soon you'll have little brown seed pods. Grab and pull and the seeds come off in your hand. Drop them into the pot and they'll sprout. No problem! : )

"They all get full sun and I never let the soil dry up."

While you don't want to keep your herbs in dry soil for too long, most herbs seem to appreciate you letting the soil dry inbetween waterings. More plants die by overwatering, than by underwatering.