The aromatic oils in plants contain the same sorts of chemicals like terpenes as in cannabis, and many are known to be pharmacologically active, including mental effects. Myrcene, for instance, is abundant in most cannabis and also found in verbena, lemon grass, hops, mango, West Indian bay tree, and cardamom.

For example here's a paper about the psychological effects of mint oil: Volatile Terpenes and Brain Function: Investigation of the Cognitive and Mood Effects of Mentha × Piperita L. Essential Oil

Another one...
Therapeutic and Medicinal Uses of Terpenes

As Tim Minchin said:

Do you know what they call alternative medicine that's been proven to work?

Medicine.

Not bullshit but not magic either.

For example, L-Theanine is psychoactive, lots of clinical support for that.

Additionally, I would say that the ritual around "sitting down and having a nice relaxing cup of lemon & ginger tea" has some calming benefits in itself - even if those benefits are not from the ingredients in the tea.

Whatever the heck lemongrass actually does, it sure tastes delicious

A worthwhile thing to keep in mind whether it's for tea, supplements, or whatever, is that medication is based on things that were observed from "natural" sources.

For example, willow bark was/is/has been used for pain and inflammation. It also contains salicin which is similar to acetysalicylic acid, which you'll know better as aspirin.

Does that mean all the things people say do a thing work? Not at all. Do some of them have varying degrees of effect of some percentage of people? Yes.

Like all of the supplement industry, it will depend on a couple things. One, what are the regulations in your country? How well are they enforced?

In the US, there is next to no regulation or enforcement, so often these things don't contain what they claim to, or not the amount claimed. So you're looking at third party testing groups or just trusting the manufacturer. Mostly the adulteration isn't harmful, but generally inert. So if there's very little or nothing there, it's most likely placebo.

Next - even if you do get the dose of what it claims - I think it's still very likely placebo, extremely weak or extremely variable effect. "alternative medicine" that has consistent effects on most people every time even when they don't know what they're taking isn't usually alternative anymore - we just call it medicine.

All that said - I personally don't see any issue with using a placebo for psychological issues - "it's all in my head" so applying a "just in my head" fix seems reasonable as long as it's working for me.

In case I'm perceived as biased or part of BigAntiCalmingTea, I'm consuming a cup right now...

Lots of comments talking about product quality and research, and that's totally true. However bear in mind that when it comes to infusions, the effects are likely to always be mild, and you also have a lot of personal taste influencing your reaction.

Take for example camomile, which has anti inflammatory properties. But how much camomile are you consuming with a single cup of camomile"tea"? Not much; also, the effects are topical ( on contact), and a drink usually doesn't stay for too long on the same spot of your throat. A cup of camomile isn't going to make a massive difference if you have inflammation on a tooth gum or your throat, and it won't do anything for inflammation on your knee. And if you dislike the smell and taste, that's not going to help at all.

I just googled it and it says this:

Amino acids and fragrance substances in tea can calm us down. In late night, a calming tea can help us sleep better. Theanine's calming effect is the long-term result of tea-drinking. The higher theanine content can effectively suppress caffeine's stimulation in our nerve system.

Placebo effect is real.

Maybe it is, but that shit really helps me

A lot of them are just subjectively calming your mind, I think, by being warm, soothing and pleasant. The main exception for me is chamomile, which I have seen some minor results with. Which is slightly annoying because I don't particularly like the flavor. At all.

It doesn't help me stay asleep though, just fall asleep. Can work sometimes when I've had a little too much caffeine.

It depends completely on the plant, the processing (if any), and other factors. I know nothing about lemongrass but St Johns Wort for example is GABAergic, acts on serotonin and dopamine, and under controlled trials shows clinical significance.

Placebo is one of those factors and it can be a MASSIVE factor. Conversely, nocebo, the opposite of placebo shows that if patients do not believe something will work, even though it's shown highly significant clinical success, chances of it working on them are far less.

I haven't noticed lemongrass calming me but I like the flavour though I'm generally pretty calm already.

Are you talking about specifically tea, or are you talking about tisanes, in general?

I remember going to a tea shop years ago. The person working there asked if I wanted one of the samples they had. The conversation went like this:

"It's good for heart health, your liver, and getting rid of toxins."

"Does it taste good too?"

"Oh yeah. Of course."

"Ok. Let's start there."

Tea can do many things but it's not medicine. That's stupid.

Some work; some don't. Google Scholar is a good way to find out whether testing's been done. Here are 2 less time-consuming ways 1) https://ods.od.nih.gov/factsheets/list-all/ 2) https://examine.com/ The other thing to watch out for is whether the bottle you're buying actually contains what it says it does. The only verifying organizations I know of are NSF and USP. Manufacturers have to pay for those lab tests, and if they do, they post NSF or USP logos on the bottles. If you just look for "natural" or "organic," you won't find anything about the presence or absence of the supposedly active ingredient. As jmp242 said, there's no regulation in the USA, so there's a lot of snake oil.

By personal (yet anecdotal) evidence, I can testify that at least camomile works. The regular tea is not concentrated enough for me to have any noticeable effect, but the oily extract (apigenin) does wonders for my stressed-out brain!

Give skullcap a try if you can find the extract or a tea, that one worked super well for me to help get to sleep