Field thistle is a robust annual and member of the sunflower family. It grows to monster proportions (easily 2 m) with good soil and proper moisture. Flowering heads bear elongated, purple to lavender disk flowers that bloom in July and August. Leaves emerge individually along the entire stem. Leaf margins, the tips of leaf lobes, and parts of the stem all bear spines. The plant dies off in early September and dries down to low mois­ture content within four weeks.

One plant has the potential to produce up to 5200 seeds in a season, but the average seed production is about 1500 seeds per plant. Seeds are dis­persed primarily by wind.

Thistle grows in a wide variety of soils, including sand dunes, but it is most abundant in clay soils. It can tolerate saline soils and wet or dry soils, but it grows best in dry soils. There are many other varieties, including Canadian and Bull.

In large thistle plants, the stem can be 3 cm in diameter. Although this stem size is admirable from the prospect of bulk, it may make cutting by conventional cutter-baler equipment difficult. The stalk consists of a large, spongy pith that does not rot or dry up for up to 1 yr. Although the stem diameter is impressive, the density of burnable mass is only moderate and the expected yield/acre is low.

Although thistle does thrive in sandy soil, most varieties of thistle are highly competitive and in some areas are classified as a noxious weed. It is also referred to as a "highly disruptive exotic plant."