The seed packet directions seem simple at first glance: sow when all danger of frost has past. Your neighbor sets out onions on her birthday. The almanac talks about old planting practices based on a waxing or waning moon. Further, it has rained every single day for the last three weeks. Simple directions have become increasingly complex. Frustrating for the home gardener, the effect of planting dates becomes a critical issue for commercial farmers and food producers.
Resources for Farmers
The study of how planting dates affect crop yield are a frequent area of study for the USDA and state departments of agriculture. Studies often focus on grain crops because they are critical to feeding both humans and animals. Field trials of planting dates may include how different dates impact on watering, crop health and insect or disease avoidance.
Resources for Recreational Gardeners
Home gardeners have a number of planting-date resources at their disposal. Best known is the USDA Plant Hardiness Zone map, which emphasizes the length of growing seasons and expected maximum cold temperatures for all geographic regions of the U.S. Less well known but equally practical is the American Horticultural Society's Heat Zone map, which projects the likely amounts of time plants will endure high heat stress in varying regions (this map may be used more for plant maintenance than choosing planting dates but serves as an excellent reminder to get plants established and chores like watering set-up and mulching done before heat arrives). Farmers' Almanacs continue their long popularity, not only with old-time rural gardeners but with suburban newcomers as well. Their weather projections, moon-planting and celestial planting schedules reflect old wisdom about choosing the best days to plant; and, to some gardeners, the old ways remain the best ways.
Effects of Planting Dates in Home Gardens
One of the best ways to choose planting dates for your garden is to find out everything you can about local considerations and conditions. Contact your County Extension for anticipated problems with insects (like cucumber beetles), disease (like last year's tomato blight) or other issues likely to affect growing conditions in your garden. Even urban extension units serving mostly city gardeners keep tabs on the regional agricultural picture and may well have good guidance on when to start bean-planting this year. Rural agents may have up-to-date information on regional field trials relating to your vegetable garden or backyard orchard.
Tailoring Planting Dates to Your Garden
Your own growing conditions can have an effect on planting in your garden. Take into consideration your own micro-environment and the factors that cause it. Large paved areas, big outcroppings of rock or the absence of trees tend to bring warmth to your soil. At the same time, the front hedge and street trees cool the front yard except for a corner sun-spot. Planting dates depend on knowing the warm, cool, wet and dry spots in your garden. If yours are the first tulips blooming in the neighborhood, perhaps your peas go in earlier than your neighbor's. Study your yard and learn.
Choosing Dates with Confidence
The best support for your confidence in choosing planting dates is to keep a yearly garden log or journal. These can be elaborate, noting daily temperature and rain, birds and butterflies observed, and the progress of new annual seedlings. Or it can be as simple as the date you finally got the zucchini in and how much you picked. You can record small trials of seeds vs. seedlings and the summer dates you tested to get a second harvest before frost. A single year's log may not help a lot; several years' worth, however, give you lots of information and reveal patterns specific to planting in your garden.