Lavender plants come in three main types: Stoechas, Spicas, and Pterostachys. Which type a lavender plant is in seems to be determined by the kind of flowers it has. There are also the Lavandins, which is a generic term for a hybrid mix of two different types of lavender.
Many lavenders are also named for their country, which sometimes (but not always) suggests a different variety.
The most common Stoechas lavenders:
The most common Spicas lavenders:
Some common Pterostachys lavenders:
Lavandins are lavender hybrids, and are sometimes listed as Lavandula hybrida or Lavandula intermedia. Many of these resulting plants are sterile and can only be propagated by cuttings.
Some common lavandin plants are: Provence, Grosso, Hidecote Giant, White Spike, and Woolly lavender.
Lavandins tend to have high resistance to fungus, and are more tolerant of adverse garden conditions. If you've had trouble growing lavender in the past, you might consider one of the lavandins.
There are lavender plants ranging from the dwarf lavenders (6-12" tall and wide) to the giant lavenders (4 feet tall and wide). Check the variety you want to plant as far as how big it's going to get.
The flower and foliage color can vary widely as well depending on variety, and of course different types of lavender have different flower shape and size as well as I mentioned earlier. Some varieties have different shaped leaves as well.
The smell (and taste) of each variety differs also, so make sure you smell and taste your plants if possible before buying to make sure you're going to like them.
Lastly, the cold hardiness of the lavenders varies widely: some varieties are hardy to USDA zone 5 while some are only hardy to USDA zone 9. Make sure you investigate the lavender variety you're considering fully before you buy.
If you really want to know everything about every variety, this website is a good resource.
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