Answer: What you describe sounds like good old-fashioned dodder. It's weird in that it germinates from seeds and then becomes parasitic on the host plants. It doesn't need roots once it attaches itself to other plants because they supply all the food and moisture it requires. Pull the dodder off your plants as soon as you discover it. This will keep it from flowering and setting seeds.
If it is in ornamental beds with woody plants, you can treat the soil with a pre-emergent herbicide such as WeedzStop (an organic product with corn gluten as active ingredient). This product is sold by Gardener's Supply Co. (www.gardeners.com; ph# 800/863-1700). You will need to reapply it every so often throughout the summer to keep the dodder at bay.
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