What do I mean when I say greens? For me it is any leafy green vegetable that you grow mainly to eat the leaves. Collards, Kale, Spinach and lettuce all fall into this category.
There is no doubt that many of the vegetables grown in the cooler nights of the fall taste better. I remember my mother saying I am waiting till after a frost to pick my collards. The reason is that plants will concentrate their sugars in the leaves when it gets colder. This will not make them taste sweet like an apple but it definitely improves the taste. The timing of planting your greens is important because you can not wait until colder days of fall to start your fall greens. They need to be planted in late summer so that they have time to grow and mature while the temperatures are warm and then they will be ready to harvest in the cool of the fall. My garden is in Georgia and our first frost usually comes in mid to late October. If I look up the days to maturity for greens most will be 50 to 60 days from plants and add 30 days if you are starting from seed. I count backwards from our average first frost date which is mid October. That means I need to put plants in the ground in mid to late August. Where you live may be different but you can do the math if you know what your average first frost occurs and you look up the days to maturity in your seed catalog or seed packet. In Georgia we can grow many of our greens through most of the winter. With a little protection when the temperatures dip below 20 degrees F my plants will last all winter. I pick the individual leaves instead of harvesting the whole plant except some of the lettuces.
Many of your fall greens are susceptible to leaf eating caterpillars. Those pretty little white or yellow butterflies that you see in the garden are laying eggs and the eggs will hatch into hungry caterpillars that will devour your greens if they are left unchecked . I have two organic remedies to combat the caterpillars. The first is called BT and this is a naturally occurring bacteria that only kills the caterpillar and is safe for humans. The second is spinosad, also an organic control but toxic to more insects than just the caterpillar. There are few other pest beside the occasional aphids and those can be controlled with insecticidal soap.
Most greens will grow better in rich organic soil amended with lots of compost and some extra organic fertilizer. Keep greens evenly moist with biweekly waterings if it is not raining and more often when they are first planted in late summer. Plant them in full sun if possible but they will take a little shade with at least 4 or 5 hours of sun.