While driving here in southern Arizona, I will occasionally see what looks like heaps of orange or yellow string draped over plants along the roadside.

Dodder (Cuscuta sp.)

Unfortunately for the plants covered in it, it is nothing so benign as string. These tangled orange or yellow heaps are actually Dodder (Cuscuta spp.), parasitic plants that can weaken or even kill their host plants. Dodders are true parasites and obtain all of their water and nutrients from their host plants, and thus dodders do not need to have green chlorophyll for photosynthesis (converting light energy to food). Dodders come in various shades of yellow or orange depending on the species.

Dodder seedlings sprout in the ground like other plants, but the seedlings must soon find a host plant because their roots and cotyledons are only able to sustain them for a few days or so. The dodder seedlings send out vining tendrils in search of a suitable host plant, and once they find one, the dodder tendrils wrap around the host plant and insert haustoria (modified roots) into the host tissues. The dodder's soil roots then die and the plant disconnects from the ground. From that point forward, the dodder is completely dependent on the host plant. As they grow, the dodder vines will continue to form more attachments to their host plants.

Dodder (Cuscuta sp.) flowers

Dodders produce clusters of small, waxy, usually cream-colored flowers. The flowers are followed by up to thousands of tiny, hard seeds that can survive in the soil for many years. This can make dodder rather difficult to control in cultivated areas because it keeps sprouting up again year after year. Cultivated plants infested with dodder should either be removed or the dodder-infested branches pruned before the dodder vines flower and produce their long-lived seeds.