Organic seaweed with plenty of umami?

Can I ask you a question? What exactly is "organic" kombu, and how would it differ from non-organic kombu?

What are they doing/not doing in order to make a seaweed "organic?" How does someone grow something in the ocean organically?

If the local product is failing to meet your standards, why not just use the regular stuff? Dried seaweed is shelf stable, so freshness isn't a factor.

Different types of seaweed have different levels of MSG, so your local stuff might be a different type of seaweed that just doesn't have the same "oomph." You may have better luck trying out some different types of local seaweed...maybe one is better than the other.

I have to say that the organic label sounds like a dumb marketing gimmick...for kombu I would just use the best I could get my hands on (that fit within my budget) and focus on more important things.

Alternatively, if you MUST use local seaweed maybe try to find umami in other ways...fortify with soy sauce, miso, or add some dried mushrooms--though all of those will change the flavor.

I would indeed be a little wary of a Chinese-made product, because you don't know what additives and such might be used in the processing. You also don't know where they got the seaweed including whether it might be growing in irradiated or otherwise seriously polluted waters. But Japanese products aren't all the same either.

In the US (or probably the UK as well), your best bet is to buy a Japanese-made product from a reputable dealer, packaged somehow that avoids a lot of moisture and air. There are a lot of these, and it's not actually going to make a heck of a lot of difference to the end-product: none of these kombu are all that good, but they're not bad either. Just ordinary kombu, perfectly fine, nothing special. If they give a region of origin, do try to avoid the northeast of Japan: you don't want to be anywhere near Fukushima, after all, and bear in mind that the Japanese government has decided, long since, that there is no problem with radiation anywhere near Fukushima, not pollution, nothing to worry about, pay no attention to that man behind the curtain....

Kombu should be brushed lightly with a dry paper towel or the like just before use, to remove dust. Do not scrub, wash, etc. Do not attempt to remove white powdery stuff unless it brushes off very easily like dust.

Once you have your kombu ready, the trick, in my opinion, is to use modern methods for extracting the flavor and umami. You need a fairly soft water -- if your water is hard try cutting it 50% with distilled water. Extraction is ideal around 140F (60C). One extremely effective method, if you have the equipment, is to put a gallon of soft water and a couple good pieces of kombu into a heavy freezer bag and seal, then sink the bag in an immersion circulator bath at 140F for 2 hours. Beyond that you won't get any real effect. I'm sure you can think of other methods. Whatever you do, do not let the broth come close to a boil: the kombu will get slimy and can take on a bitter flavor. If you extract at 140F for an hour-ish, you'll get all the good stuff out of the kombu and your results will be very much as though you'd used a much better kombu in the more traditional quick-in-the-pot-until-barely-simmering method. (Note that this method was invented or at least broadly documented by Murata Yoshihiro of Kikunoi, Kyoto.) If you're making ordinary dashi, with katsuobushi, be sure to remove the kombu before heating to a bare boil and adding the flakes.

Bear in mind that the balance between kombu flavor and katsuobushi flavor, and their total strength in the dashi, are very much regional phenomena. I gravitate toward maximal kombu and moderate katsuobushi, because I lived in Kyoto for 2 years and that's kind of how they prefer it. Tokyo would tend toward less kombu and more katsuobushi, on the whole, and they seem to make more use of nibandashi as well. For 2 liters of water I'd use a standard strip of kombu about 3-4 inches by about 12 inches, and about half a big bag of katsuobushi, resulting in a very intense dashi that would work beautifully for suimono (clear soup). For miso soup you could certainly use a lot less of each, because the miso will cover a lot of crimes.

Good luck!

While we're on the subject, I'd like to share a product recommendation. I had this suggested to me when I lived in Japan. It's one of many brands of dashi-makings that comes in what amounts to a teabag. You put the bag into water and bring it to a strong simmer and it's ready. I tried several brands, and this one was definitely the best for me (apart from one "gourmet" brand that cost a small fortune and, though good, wasn't to me worth the money). I have shifted to a long, slow simmer method -- bring to a gentle simmer (140F) and turn the burner very low, which in my pan on my range will hold it at a stable 150F, where I leave it for an hour or so.

I mention this because my wife recently spotted this very brand at Wegmans, which means it's become available in the US. I do not, unfortunately, have a photo of the packaging in its US form, so you'll have to look for something that matches visually. The package shown contains classic katsuobushi-dashi, but they also make kombu-dashi, niboshi-dashi, and I think some regional variants.

In the first photo you can see what it looks like when made strong enough for my (Kyoto-trained) palate.