Plants with yellow flowers are a common sight in roadsides and other grassy areas as one drives across Iowa at this time of year. There's a good chance these plants are one of the weedy mustards (wild mustard, yellow rocket, hedge mustard, etc.), golden alexander, or wild parsnip. The mustards are more likely to be found in disturbed areas where the sod is not very competitive than the other yellow flowered plants.
Golden alexander and wild parsnip are both biennials in the Apiaceae family (carrot), but have completely different heritages. Golden alexander is a native prairie plant and most likely to be found in restored prairies. It is one of the hardier native forbs and shows up in roadsides and other areas where mowing or spraying are not used. Wild parsnip is an exotic species that thrives in roadsides and other grassy areas. It is a plant to respect because all parts have a toxin that hypersensitizes skin to UV light, resulting in severe blistering and scarring. Concentrations of the toxins are much higher in wild parsnip than in the tame varieties.
While both plants form basal rosettes of pinnately lobed leaves, they are very easy to distinguish from each other. The lobes on golden alexander leaves are pointed, whereas wild parnsip have egg-shaped lobes. Wild parsnip leaves typically have many more lobes than its yellow counterpart. Golden alexander flowers a few weeks earlier than wild parsnip; golden alexander is in full bloom now whereas wild parsnip is just beginning to flower in central Iowa. Flower stalks of golden alexander typically are 1 to 2 ft in height, while wild parsnip reaches heights of 4 ft. Like all members of the Apiaceae family, their flowers are arranged in an umbel, an infloresence that resembles an umbrella in structure. Poison hemlock and wild carrot are two other weedy plants in the Apiaceae family.
So if you encounter a yellow flowered plant as you enter a field, take time to determine exactly what plant has brightened your day. If it happens to be wild parsnip, be sure to keep your distance.