Answer: Both zucchini and cucumber are warm-season crops, are usually planted in the spring after the soil warms up. They grow best in a sunny spot where they have room to sprawl, in soil that drains well. Most veggies tolerate a soil pH of 6.0 to 7.5, so pH isn't a problem with your plants. Peatmoss will help acidify the soil, and as long as it's mixed well into the soil, it's a great source of organic matter. I suspect the nighttime temperatures are too cool for the plants to thrive, and that's why the leaves are yellowing. Overwatering will cause similar symptoms in the early summer. Instead of growing warm-season crops at this time of year, why not replace your ailing squash and cukes with lettuce or spinach, or try growing cabbage or carrots?
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