You might not realize it, but if you’ve ever asked questions like: “I have pink/orange/yellow yarrow. Can I substitute it instead of the official white Achilles millefolium?” Or
“My echinacea doesn’t look like the pictures of E. angustifolia or E. purpurea. It’s lots of different colors! Can I still use it?”
You are probably asking about whether you can substitute one cultivar of an herb for another. But you may also be looking at a hybrid, or a different species altogether. . .
A cultivar is a strain of a plant that has been selected over time to emphasize certain qualities. To use an analogy, German shepherd dogs in the United States tend to be slimmer, smaller versions of their stockier German-bloodline counterparts. Just like a dog breeder might favor some traits over others within a breed, garden aficionados sometimes create new bloodlines, or cultivars, of plants for beauty or practicality. New colors, extra showy blooms, or increased drought tolerance are popular reasons to create a cultivar.
But what about hybrids? Just like a dog can cross with a wolf, resulting in a wolf hybrid, garden aficionados sometimes create new hybrids of distantly related plants. Rosa damascena, the Damascus rose, is a great example of this. R. damascena is actually a cross between several different "breeds" of rose, and yet it is valued for its fragrance and the essential oil made from it.
And then there's the question of whether your plant is the same species as the official species used in herbalism. Are you looking at a dog or a wolf? (And before you laugh, some breeds of dog can look surprisingly rangy and wolf-like!) Take yarrow as our herbal example. There are around 130 different species of yarrow, some of which are medicinal and some of which are ornamental. The official species used by herbalists is Achillea millefolium, but your yellow yarrow may actually be Achillea filipendulina, which is an ornamental.
So if your plant is a different species, you will need to carefully research that species to determine whether it can be used interchangeably. If you plant is a hybrid, you will also need to do some research, but chances are (unlike the damascus rose noted above) it won't have a history of herbal use, especially if it is a modern hybrid. That leaves cultivars. Should you use a cultivar in place of an official variety?
I wouldn't. Here's why:
When changes happen to a plant’s appearance, chances are good that other things have changed, too. You may be able to observe some of the changes easily (color, fragrance, hardiness) or not (potential changes in plant metabolism that alter safety or effectiveness).
Yarrow is a great example. I have two types of yarrow in my garden. One is the “official” wild, white yarrow. Here’s how it compares with the pink, domestic yarrow cultivar:
Pink yarrow: blooms for a short time in midsummer. Dried leaves and flowers lack the signature yarrow scent (or any scent, really). Grows in a tidy little clump that keeps to itself. Pollinators are not that enthusiastic about it, deer and rabbits leave it alone.
White yarrow: blooms almost year round (short break mid-winter). Dried leaves and flowers have a strong, characteristic fragrance. Grows abundantly and with vigor. May bully and crowd out other plants if left to own devices. Tolerates heavy harvesting with apparent glee, taking it as a personal challenge to grow back heavier and thicker and conquer more garden real estate. Deer and rabbits browse occasionally, but not heavily. Pollinator species adore it.
Echinacea is another that’s especially common to find as cultivars right now. Some of the new varieties are stunning (like these over at Burpee Seeds), and they are very popular as garden plants because of their beauty and drought tolerance.
Finding the Right Varieties
When you are at a garden center or shopping online for seeds and plants, look at the plant’s botanical name. If you see a plant name followed by another name, you are looking at a hybrid. An example for yarrow might look like: Achillea millefolium, ‘Cherries Jubilee’.
It might also be written like “Echinacea, Warm Summer” or “Yarrow, Cherries Jubilee”. A patent number is another dead giveaway that you are looking at a cultivar.
To make sure you are getting the original varieties of an herb that are known for their beneficial properties, make sure to look at the botanical name and match it with the one recommended in a good herbal reference book. (You can read more about Learning Botanical Names in this article that I wrote for the Academy). If you aren't confident about whether the plant you are considering is going to work out for your herbal intentions, shop for seeds and plants online at nurseries that specialize in medicinal plants to help cut out the guess work!
All the best,