The most common meaning of the word “Weed” is an unwanted plant that poses a threat to more desirable plants.  But on the farm it’s not always easy to tell them apart from crops.
Tomatoes, peppers and potatoes are all in the nightshade family.  Purple nightshade is also a very common weed on our farm.  When it is small, it looks very similar to its food-crop cousins.  This allows it often escape the gaze of workers hoeing around tomatoes or potatoes.  Once it gets full-size, it is easily recognizable due to its tiny purple berries.  But it is also tremendously hard to remove by them, having established large and healthy roots similar to those of a tomato plant.
Beets, spinach and chard are all members of the chenopod family.  Several species of this family are also among the most common annual weeds in California:  lambsquarters and Pigweed Amaranth.  The tiny seedlings of all these plants appear to even the trained eye as almost identical.  Unfortunately, the wild cousins grow much more aggressively and quickly than their domestic relatives.  By the time it’s easy to identify which is which, the weeds have often completely taken over.
Sometimes it seems like we should be eating the weeds instead of the crops.  But while they are technically edible, Lambsquarters have enormous amounts of oxalyic acid that make them unpalatable, while Amaranth has fuzzy leaves that verge on prickly.  Both have tough, woody stems.
One of the most persistent and difficult to control weeds in California and many other states is Field Bindweed, a member of the Morning Glory family.  This is a perennial weed whose roots live deep underground where they are nearly impossible to eradicate, even with the strongest herbicides.  Its vines can grow 10 feet, sprawling over the ground or twining up other plants, stealing both their sunlight and their water.
Humans have adapted several strains of morning glory, taking advantage of their prolific nature to provide food crops.  In tropical regions, morning glory varieties are eaten cooked like spinach and have a very similar flavor.  There are also dozens of ornamental varieties.  And here in California as well as numerous other states, farmers grow a type of morning glory whose roots swell just below ground to produce sweet, edible tubers with enormous nutritional value.:
Sweet Potatoes grow just like their weedy cousin, but with much bigger leaves.  They have few pests and require little fertilizer and water while producing truly tremendous yields.  The only problem with sweet potatoes is a significant one:  getting them out of the ground.  Unlike potatoes or carrots, whose edible portion grows just below the plant in a fairly compact area, sweet potatoes make long, wandering roots.  The tubers can reach down more than 2 feet below the soil and as far as 3 feet away from the plant.  This makes it more time-consuming and complicated to get them out of the ground without damaging them.  It also limits the types of soil where they can be grown commercially.
We use the same harvesting machine to harvest both potatoes and sweet potatoes.  But while we can harvest an acre of potatoes in a day or so, it takes 3 times as long to harvest the sweet potatoes.  And even with the digging machine set as deep as it can go — about two feet down — we still routinely break tubers that have grown down even deeper than that.
Unlike the roots of field bindweed though, the sweet potato tubers we leave behind after harvest will not resprout next year and grow anew.  That’s the only thing that keeps sweet potatoes from being a weed.