Trifolium incarnatum — Crimson (not red) clover.
I had always heard the clover in the photo at right referred to as red clover. It's good used as an cover crop in agriculture and in the ornamental landscape to improve the soil while you are transitioning an area from one purpose (like lawn) to another (like a garden bed).
Recently I was reminded why we need to learn and use botanical names for plants rather than common ones.
A friend asked to be reminded what clover to use as a cover crop. I said red clover—thinking of what one of my contractors installed for another client that looks brightly red as in the photo above. It's red! He used the word red, so I used it, too.
Many people use plants' common names, and when I know a friend or client isn't well versed in Latin botanical terminology, I try to use the common names, too. I figured if my contractor asked for red clover, and got what he wanted (top photo), then it would be the same universally.
Trifolium pratense is commonly known as Red clover, but you can see the flower is more pink or purple than red. Don't confuse it with Trifolium incarnatum, common name Crimson clover, truly has RED flowers.
My friend went to the nursery and asked for red clover, but got what is pictured in the second photo (at right), which has a much more pink flower. It's common name is Red Clover, yet, you can see it is not really very red at all. In the first photo, the clover's common name is Crimson Clover (yeah, like the song—sorta). Very subtle difference in name, yet there are significant differences between these plants.
I am finding that nurseries and garden centers are embracing their customers' new desire to be more sustainable. This often includes planting cover crops in our residential gardens. The customers come in asking for bulk red clover because that is the color of the blossoms they have seen in pictures in articles they may have read.
So the nursery provides them with Red Clover (Trifolium pratense). Yes, this clover will do the job of adding nitrogen to the soil and blasting through clay soil like a champ. And actually, it IS used a great deal in agriculture, particularly when the field it's used on needs to be covered more than just one year.
The problem for we residential gardeners is that Trifolium pratense (Red Clover) is a perennial, and in our mild zone 8 climate (where I garden, design and teach) it can easily invade other areas by underground rooting. And it can be difficult to control and remove without chemicals. Some would call it invasive.
The plant in the top photo is Crimson Clover (Trifolium incarnatum)—an annual in our zone, and it does its job of improving the soil in just one growing season. It then dies, after which you mow it down, till it under (or just mow then leave the dead foliage to compost in place), then plant what you want to plant in its place. It doesn't invade by underground roots—at least not in our Zone 8 from what I'm reading. If anyone knows differently, please let me know.
Crimson Clover (Trifolium incarnatum) is what you need to be asking for if you want the truly red-flowered stuff that is an annual.
Seed for Trifolium incarnatum (Crimson Clover) can be bought at Territorial Seed. And if you buy from a garden center, be sure the package says the botanical name (Trifolium incarnatum) on it and not just 'red clover'.