There are three main methods to determine when vegetables should be planted. They all have their merits, and in fact it's very helpful to use more than one. Here they are:
Work from Last Frost Dates:
In order to work from last frost dates, you need to know the last frost date for your area. That is, what is the date after which there is only a 10% chance that there will be a hard frost? Note, in other words, that this date is not cast in stone and in about one year in ten you can expect it to be wrongity-wrong-wrong-wrong, in the wrong direction. Furthermore, it's likely that different sources will disagree on what that date is for your area. And beyond that, microclimates have a big effect on last frost dates. For example, we live in a largish microclimate in a valley by a large body of water. In spite of its size, it rarely appears on zone maps as they are hardly ever that detailed. But even beyond that, there are micro-microclimates. Things grow very differently on the south side of our black-sided garage, in well drained sandy soil than they do down in the wet field by the woods.
The best way to determine your local Last Frost Date is to ask around, and get the consensus of your gardening neighbours, the closer the better. They are the ones who will have tested it over time and know the answer as well as it can be known. In general though, for southern Ontario the last frost date is likely to fall somewhere between May 15th and May 21st. I use May 15th; it is both reasonably accurate for me and conveniently located in the middle of the month, making calculations easy.
Which is good, because the next step is to make a chart. Make a list of all your seeds that you will be planting, with the information about when to plant them in comparison to the last frost date, and whether that is indoors or outside. Most good seed companies will list this information, either on the seed packet or on their website; otherwise, the information will usually be fairly easy to find on-line. For example, tomatoes and peppers need to be started indoors, 6 to 8 weeks before last frost date. I find peppers are generally slower to get going than tomatoes, so I would start them 8 weeks ahead, and tomatoes 6 weeks ahead. That means I should be planting my pepper seeds on March 15th and my tomatoes on April 1st. They then get planted out "after danger of frost has passed", or sometime between May 21st and June 1st, depending on how conservative I want to be about it, and also how able I would be to cover them up and keep them warm if there was one last frost after they went out.
Most of the advice given is a bit on the vague side. You really need to do your own research and make your own decisions about when to plant. If you are willing and able to cover things to protect them, they can go out earlier, and the more effective the covering the earlier they can go out. Obviously this is a lot more work, and you need to assess whether it will be something you want to do.
I treat the months as having 4 weeks when I calculate planting dates using the Last Frost Date; again, not particularly accurate but close enough for private sector work.
Sort of like salt and pepper, the First Frost Date is generally given as well when you are looking for the Last Frost Date. This is the date of the earliest (average, expected, more-or-less) frost in the fall, and it is as reliable as the First Frost Date, in other words not very. But it too will be useful in calculating last planting dates for fall crops, and last harvest dates. I call mine October 15th, which is grossly inaccurate and I know it; it is not unusual to get a light frost in the last week of September. However, there is usually then a big gap before the next frost which is indeed usually around the middle of October and a much harder one that will kill things even if they are covered. I can generally cover things up for that first frost and have them keep going until mid October; even the tomatoes, so I use the later date. This is all a matter more of pragmatism than scientific accuracy.
Actually, once I wrote all of the above I remembered that Mr. Ferdzy was all excited last week, when he found that Johnny's Selected Seeds has already written that chart for you; all you need to do is type in your Last Frost Date here, and Bob's your uncle. Or maybe that's Johnny.
Measure Soil Temperatures:
It isn't just the air temperature that affects how vegetables grow. The temperature of the soil is at least as important, if not more so. Some things will germinate in very cool soil, such as peas; others will not, and will not grow at all unless they are very warm, such as sweet potatoes. You can get inexpensive thermometers to measure your soil temperature; we just use the one that we bought as a compost heap thermometer, as it has a nice deep probe. You want to test about 6" to 8" down, in general. The following chart is a good approximate set to use:
- 5°C - Plant spinach, kale, lettuce, bok choi, parsnips, peas, radishes, fava beans
- 10°C - Plant Chinese cabbage, leeks, onions, Swiss chard, turnips, potatoes
- 15°C - Plant beans, beets, brassicas
- 20°C - Tomatoes, eggplant, peppers, cucumbers, squash, melons, corn, peanuts, sweet potatoes
That's phenology, not phrenology, which is completely different. Unlike phrenology, which most people have heard of, phenology (which most people haven't heard of) is an actual scientific thing. It means the study of the timing of natural events; in particular, how they relate to each other. So, for example, we look for the first big flush of dandelions to bloom - which happens when the soil is about 10°C - and plant our potatoes then.
There are all kinds of little observed correlations that gardeners pass around for when to plant different vegetables; some of them, like the suggestion that when the oak leaves are the size of a squirrels ear it is time to plant corn, are a bit precious. What kind of oak? (What kind of squirrel? What kind of corn?) Others are more useful; and it doesn't take too long to figure out that most of the suggestions line up with soil temperatures. So, somewhat tentatively, I'm going to put them out as:
- 5°C - crocuses bloom, maple trees flower
- 10°C - dandelions, forsythia bloom
- 15°C - daffodils bloom (not the earliest ones)
- 20°C - bearded iris and lilacs bloom
If anyone has a better signal for the 15°C soil temperature than the daffodils, I'd like to hear it; also any other signs you use to determine planting time.