From your description, I think you mean wild parsnip (Pastinaca sativa). This is a biennial plant that originally came from Europe and Asia and has become naturalized in the U.S., especially in abandoned fields, along roadsides, and in open disturbed areas. Its umbels of white/yellow flowers bloom from May to October, so the season is very long, with lots of seeds. It can cause burn-like reactions, especially bad when weed whacked as bits of the plant fly off and come in contact with and irritate the skin. The chemicals that cause the problem are called psoralens and they cause phyto (plant)-photo (sunlight)-dermatitis. Wild parsnips are related to edible parsnips. If you have a lot of it on your property, I would advise getting thoroughly covered up and then mowing the plants down, before applying a weed killer such as Round Up. If you don’t want to use chemicals, smother the area with black plastic and let the plants cook for a season at least during hot weather. Remember to clean any equipment thoroughly afterward wearing rubber gloves, etc.
