The term “thistle seed” is confusing because it refers to two distinct types of seeds with vastly different lifespans and uses. For most people, it means the tiny black seed used to feed birds, but botanically, it also describes the seeds produced by noxious weeds. The longevity of the seed depends entirely on whether it is a commercial product meant for consumption or a wild seed designed for long-term ecological persistence.
Clarifying the Terminology
The small, black, oil-rich bird feed commonly sold as “thistle seed” is actually Nyjer seed, which comes from the plant Guizotia abyssinica. This plant is not a true thistle but a species of the sunflower family primarily cultivated in Ethiopia and India. The misnomer “thistle seed” was used historically because finches enjoy feeding on the seeds of actual thistle plants.
In contrast, true thistle seeds originate from invasive weeds like Canada Thistle (Cirsium arvense) or Scotch Thistle (Cirsium vulgare). These seeds are an ecological concern due to their ability to sprout and spread. To prevent the unintentional introduction of weed species, all imported Nyjer seed is legally required to undergo a heat sterilization process. This treatment effectively devitalizes the seed, meaning the bird feed will not germinate if spilled onto the ground.
Shelf Life of Packaged Bird Seed
The shelf life of packaged Nyjer seed, which is intended for consumption, is typically six to twelve months when stored correctly. This relatively short duration is due to the seed’s high oil content, which is a desirable nutritional feature for birds but also makes the seed vulnerable to degradation. Over time, these oils can turn rancid, which makes the food less palatable and nutritious for finches.
The primary enemy to seed longevity is moisture, which encourages the growth of mold and mildew. Storing the seed in a location that is cool, dry, and dark is paramount to minimizing spoilage. Exposure to high temperatures and direct light can also accelerate the degradation of the delicate oils within the seed.
If the seed has begun to spoil, there are several noticeable cues that indicate it is no longer fresh. Visible mold or mildew growth is a clear sign that the seed should be discarded immediately. Another indicator is clumping, which suggests the seed has been exposed to excessive moisture.
A rancid or musty odor is also a sign that the oils have broken down and the food is past its best. To maximize freshness, the seed should be kept in an airtight container, which prevents air exposure and infestation by pests. For long-term storage beyond a few months, freezing the seed in a sealed container can significantly extend its quality.
Seed Bank Longevity of Weed Thistle
The lifespan of true thistle seeds is measured by their viability within the soil seed bank, which refers to the natural reserve of dormant seeds in the ground. The ability of these seeds to persist is heavily influenced by environmental factors, particularly their depth of burial. While some thistle seeds germinate quickly, others enter a state of dormancy, allowing them to remain viable for extended periods.
For common noxious weeds like Creeping Thistle, most seeds near the soil surface will germinate within the first year, or their viability will drop sharply within four years. Seeds that are buried deeper in the soil are shielded from the environmental cues that trigger germination and are protected from predation. This deep burial allows for dramatically increased longevity.
Studies have shown that some thistle seeds can remain viable for several years, with certain species persisting for decades under ideal conditions. Scotch Thistle seeds buried at depths greater than 190 millimeters have been observed to remain viable for 16 years. The predicted interval for viability to fall below one percent can range up to 50 years, depending on the soil type. This remarkable persistence means that effective weed management requires long-term strategies, as new plants can emerge from the seed bank many years after the parent plants have been removed.