Answer: It's natural for the lowest leaves of tomato plants to turn yellow. They're just old and the plant has gotten its use out of them so drops them in favor of healthy new leaves towards the top of the plant. Just pick them off the plant. If more than just the lower leaves are yellowing, there may be other problems. Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Soil should be kept moderately moist (but not wet).
Cucumber plants can be allowed to sprawl on the ground or they can be coaxed to climb up a trellis or other structure.
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