There are over 2,000 different species of vascular plants in Indiana, about 25 percent of which are non-native to Indiana. Most of them exist without issue; however, some compete with and crowd out more desirable native species, while others can even prove dangerous to people or livestock. For further information, including detection and response resources for invasive species, I would encourage you to check out the Midwest Invasive Species Information Network available here: http://www.misin.msu.edu/
Two weeds in particular that you may be noticing throughout the county that are currently flowering are poison hemlock and wild parsnip. Both are members of the carrot family and they are biennial weeds, meaning that they take two years to produce seed. Seeds currently being produced will give rise to plants that spend their first year as low-growing basal rosettes that develop a long, thick taproot. During their second year, plants bolt by producing stalks and multi-branched stems topped with umbrella flowers. Poison hemlock can reach heights of 8-10’ and produce white flowers shaped a little like an umbrella. Wild parsnip reach 6’ in height and produce similar shaped yellow flowers.
All parts of a poison hemlock plant are considered highly toxic and can cause respiratory failure and death in mammals, making it one of the deadliest plants in North America. Its toxins must be ingested or enter through the eyes or nasal passages to induce poisoning, but it is still recommended not to handle them because sap on the skin can be rubbed into the eyes or accidentally ingested when handling food. Wild parsnip sap on the other hand does produce a toxin when touched and can cause severe blistering. It can be hard to determine the cause of this, as sometimes the effects are not seen until 24-48 hours after exposure.
While these weeds could still be carefully chopped or mowed down, it is too late this season for strictly chemical control. If you do attempt mechanical removal, wear protective clothing including gloves and eye protection, and thoroughly clean these garments after leaving the area. If the plants are chopped or mowed, the taproot will send up new shoots so it must be done frequently, before they set seed. It is then recommended to take note of where they are growing and hit them early and often next spring with glyphosate or 2, 4-D while they are still rosettes or very small second year plants.