The only thing more annoying in your garden than a weed is one that pokes you when you try to uproot it. While it's smart for the survival of the weed, it's not good for your other plants or your fingertips. Sometimes, the only way to eliminate a certain weed completely is to find out what kind of weed it is and what type of treatment, whether chemical or biological, to use on it. In addition, knowing what type of weed you have can help you prevent the conditions under which it is thriving in your yard.
Take a picture of the offending thorny weed if possible. This will help you in trying to describe the weed to others or when looking through books or pictures for a match.
Examine the leaf shape of the weed as well as the location of its thorns. Some thorns are on the stems under the leaves, and others are along the edges of the leaf itself.
Note anything unusual about the weed, such as its location, any patterns or colours on the leaves or other interesting characteristics.
Examine the height of the weed itself as well as the length of mature leaves and any flowers or seeds.
Find a reference book or website on weeds, such as the Weed Science Society of America, (WSSA.net) and enter the characteristics of the weed into the search function.
Look for yellow heads at the tips of tall branches. The yellow starthistle has 1- to 2-inch spines that stick out and around the yellow flower head in the shape of a star. The plant begins in the early spring as a rosette of green leaves, each about 8 inches long and 2 inches wide, before growing up to 5 feet in height and developing spikes and flowers.
Examine a stem with hairlike bristles. While these are not exactly thorny, the bristles of a nettle do sting. The leaves of these tall, green plants with tiny flowers are edible and can be collected for boiling and eating, as long as you wear gloves while picking them.
Put on a pair of gardening gloves and pick the weed. Throw it in a plastic bag and take it into a garden centre to see if they can tell you what the weed is if you are unable to identify it.
Make an appointment with a horticulture or botany professor at a local college or university. A professor of a landscape design class may also be able to help. These professionals will have access to more books and references than you may be able to find online or otherwise on your own.
Ask your local extension service to put you in touch with a master gardener in your area. Master gardeners have undergone extensive training through the extension service and devote some of their time to helping their community with gardening matters. A master gardener in your area will likely be familiar with the types of weeds you are fighting.