The spicy, green paste you know as wasabi from your favorite sushi restaurant probably isn't real wasabi! In the United States, wasabi is mostly unused and unfamiliar outside of sushi, anyway, which would explain why it's hard for us to know or tell the difference between the real stuff and the impostor.
True wasabi belongs to the same botanical family as horseradish, radishes, and mustard, and it has similar sinus-opening heat. In fact, the fake wasabi you get alongside your favorite sushi roll is made from horseradish and mustard. It comes as a bright green paste or a powder to mimic the pale green color of real wasabi. The heat from wasabi or horseradish dissipates quickly because the heat-causing compound is more water soluble than the oily compound found in chili peppers. (That also explains why the heat from chilis burns in your mouth and throat longer — even after a big glug of water!) While the fake wasabi has similar spice properties when compared to true wasabi, there are flavor differences and subtleties that only the real stuff can offer.
Wasabi plants are difficult to grow, making them hard to come by and rather expensive, if you do. Wasabi grows naturally in rocky stream beds in Japan where they are shaded by trees. Under these very specific growing conditions, it takes almost two years to cultivate a sizable wasabi plant. Wasabi grown commercially is often grown in mudbanks. Farmers have had success growing wasabi in the Pacific Northwest of the United States, but even if you are lucky enough to find it, prices for fresh wasabi can climb to almost $100 per pound. That being said, a little wasabi goes a long way, so you don't need much.
Wasabi is often mistaken for the root of the plant when it is actually the stem of the plant, called the rhizome. The leaves of the wasabi plant are also edible. Once the root system and leaves are removed, the tubular rhizome is sold for use. To prepare a wasabi paste, the bark-like layer of the rhizome is removed and the stem is ground into a traditional sharkskin paddle or ceramic grater to create a delicate, light green paste. Fresh wasabi paste hits its peak spice and flavor about 5-10 minutes after grating. Then it begins to lose its heat after 15 minutes.
Here's the bottom line. When it comes down it, the wasabi you know and love may not be the real stuff, but it's a reasonable enough substitute considering the cost and preparation associated with true wasabi. Real wasabi has a more subtle, herbaceous flavor and heat, but the easily accessible horseradish-wasabi paste can pack a real punch of flavor and heat to give you the wasabi zing you're looking for.
If you'd like to purchase a wasabi rhizome, there are a number of online retailers that can ship you a piece of fresh wasabi. Or, if the wasabi substitute works for you, it is usually available for purchase in a large grocery store or international market as a paste or powder. Get creative with your wasabi beyond your typical sushi roll! Flavor a tuna burger with a spicy wasabi topping or prepare salmon with a creamy wasabi sauce. You can even try adding wasabi to a salad dressing made with avocados. Open up your mind (and sinuses) to the possibilities of wasabi!