Answer: As with any plant, if there is a weakness beneath the driveway then the roots will find it and possibly cause a problem. Most plants have a root system at least as large as their above ground branches network, so they do need ample room underground. Often, in a very narrow location, a vine growing on a fence is a better solution. The root problem is minimized and there is less risk of scratching vehicles (or people) when they squeeze by.
Sunflowers are usually planted directly in the soil where they are to grow. They need a warm soil and full sunlight to do well but are considered easy to grow. Sometimes, cut worms or birds will damage or steal the seedlings. Maybe this is what has been happening.
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