Two things I've heard that can be connected with lack of pungency in herbs is too little sun (probably not the issue here but you never know) and too much water.

Would either of those things apply?

I've been letting my 7 year old water the plants as he's showing so much interest in gardening with me and the hubby. Perhaps I need to monitor him a bit closer....

If I cut it back and watch the water, will it regrow slower and get that wonderful smell back?

You may want to try adding some liquid fertilizer to the waterings. Usually there are some safe, mild, fertilizers like fish based, or plant based ones that can help to improve aroma. Your wanting to get the plant to increase its natural oils, which are what makes most have a smell and taste thats going to be more noticable. Some years ago, Gardens Alive offered a fertilizer specifically blended for herbs. I found it was great to give to basil and dill, but unfortunately, they stopped offering it. A fert with a good ratio of natural nitrogen can help.

I would not add fertiliser to your lemon verbena. With many plants which have a strong perfume, the poorer the soil, the stronger the smell - because they work harder to keep going. That's how they like it. I wouldn't worry about watering it either. We're in drought in my part of the world and my herbs haven't had a decent drink for more than 6 months - summer, too, with temps around 40C. We had a bit of rain about a month ago - enough to dampen the first layer of the soil, but nothing more. The lemon verbena is going strong - and usually by this time of the year it has died down to the ground.

Basically, leave it to its own devices. If it's in shade or partial shade, it will be happier in full sun. If you move it, give it a good, deep drink, then walk away and forget about it.

Harvesthunt:

Here's another way of thinking about it. It takes a lot of energy for the plant to manufacture flavors, oils, aromas, color, etc. And I mean that literally. If we (over)fertilize our plants they can be encouraged to expend their energy resources on growing (making more foliage, more stems, more bulk) rather than anything else.

Thus, whenever some comments about a lack of flavor, or of fruit development, but tell me how huge their plant is getting......BINGO!

So, your observations are right on. Yes, accelerated growth can reduce the quality of the fragrance.

Not to be a smart-alec, but are you sure this a the lemon verbena? I don't grow a lot of them, but have never had one grow 18 inches to 2 feet in a week. Generally, it is a rather modest grower for me.

Well in the hopes it will help I will share my newbie mistake. I mixed in a few handfuls of uncomposted coffee grounds into the soil of my potted mint plant. It still grew fast, but the leaves were huge, lightly coloured, odourless (or close to it) and instead of the lovely purplish stems it had started out with it had pale light green stems. Also maybe one leaf every few days or a tad more, would show up as pale yellow and faded. I am still experimenting but based on my research it was Nitrogen deficiency. The coffee grounds breaking down actually stole the nitrogen my plant needed. So I have started giving it a half solution of 10-10-10 with every watering and if it starts improving I will take it out of the pot (it needs transplanting anyway) and shake off as much of the coffee dirt and replace it with good potting soil. So my newbie 10 cents!