Here are three tips for successfully planting your winter vegetables:
1) Plant your winter crops to mature around the time of your first fall frost.
It will take practice to learn when it's the best time to plant your winter crops. Keep a record of the days to maturity for each of your seed packets. Be aware that the real days to maturity will vary depending on your local climate and weather, and the amount of sun or shade your garden receives. The same plant variety might take only 60 days to mature in a warm climate, but 70 days in a cooler one. Plants generally mature faster during a sunny, hot summer than a cooler, cloudier one.
Take your best estimate of the days to maturity, and add about 2 weeks to it for your fall/winter crops. You have shorter daylight hours in late summer and fall, and this will slow down the growth of your plants. If I was growing a very fast maturing crop, such as a 35-day Japanese turnip, I would only add about a week.
So, if the date of your average first fall frost is October 15th, and your seed variety has a 60-day maturity, then you would plant your seeds around August 1st (count back about 74 days from Oct. 15). If the maturity date is for transplants, be sure to add yet more time to get your transplants started.
If you plan to cover your fall crops with row covers or cold frames as soon as the weather starts cooling down, you can delay planting your seed about 14 days. The sheltered climate will extend your growing season.
You don't need to be super-exact with your fall planting schedule, other than to be sure to allow enough time for your crops to grow. I usually plant most of my winter root crops by mid-July, larger winter cooking greens by mid-August, and small salad greens by mid-September.
2) Give extra care when starting your fall/winter crops in the summer heat.
It is very challenging to start cool-season fall/winter crops during the heat of summer in July and August. Some seeds – like lettuce, spinach, and cold salad – will simply refuse to germinate when the soil is too hot.
First, keep the soil moist, even if you have to lightly water the soil twice a day during very hot weather. Before you plant the bed, make sure you have thoroughly watered the soil the full depth of the bed. It's very easy to water the surface inch or two, and not realize that the soil is bone dry 3-4 inches down. Dig down into the soil to be sure!
Second, provide partial shade to the surface of the soil. There are several way to do this:
a) Spread a very thin layer of grass cuttings on top of the soil, covering perhaps 50% of the surface. You should be able to see some of the soil between the dried blades of grass. Seeds can germinate through this, but it will help keep the soil moist.
b) Provide partial shade with cheese cloth, lattice (vinyl, not treated wood), shade cloth, or other material. Be sure you give good ventilation, so the young seedlings won't overheat.
3) Provide good pest control for your young, delicate seedlings.
In summer, Insect pests are well-established and young plants are very vulnerable. Covering your plants with very light-weight row covers or window screening will help keep some pests off your plants. I use organic baits like “Sluggo®” to help control slugs and snails in my garden. If you choose to use insecticides, make sure you follow all precautions and wait the required number of days to harvest after spraying.
Keep all weeds under good control at all times. This is pretty easy to do in small raised garden beds. Your winter vegetables have a limited time to grow before cold weather arrives, and weeds will slow their growth. If your garden is new and you have the time, prepare your fall beds at least 3-4 weeks early, and keep the soil watered and shallowly cultivated until planting time. This will kill off many of the surface weed seeds. You usually won't need to take this step if you use potting soil in your garden bed.
Some gardeners find it easier to start some of their fall crops in seed flats, either up off the ground in partial shade near their house or under lights inside their home. However, most root crops can't be transplanted, and need to be directly seeded into the garden.
If your neighborhood has problems with moles and/or voles (small rodents), do what you can to protect your winter crops. My best solution was to secure hardware cloth underneath my 8-inch high raised beds. This provided soil deep enough to grow most carrots and potatoes (though not sweet potatoes), but provided excellent control against these pests. You might choose to use trapping or baits instead. Read How to Protect Your Garden from Voles