What about herbaceous plants like flowers, vegetables, and herbs? Perennial herbaceous plants that live over from year to year can be planted almost any time, just like woody plants. But annual herbaceous plants have specific temperature guidelines or requirements.
Most gardeners have become used to planting most flowers and vegetables from started plants. However, some vegetables and even flowers do just as well if not better when planted from seed directly in the ground. This is where soil temperature becomes a factor. In fact it is the controlling factor for seed germination.
Annual herbaceous plants, which grow best in the cooler weather of spring and early summer, will sprout at soil temperatures as low as 40 to 50 degrees. Annuals, which prefer the warmer growing temperatures of mid-summer, require soil temperatures of 60 to 70 degrees. It turns out that the annuals that prefer warmer growing temperatures are susceptible to damage at any temperature below freezing — 32 degrees. They do not want to have their little heads above the ground when they might be frozen. On the other hand, those annuals that prefer cooler growing temperatures can withstand temperatures several degrees below freezing (24 to 28 degrees).
This is where the average last frost date becomes a guiding factor. Cool weather annuals can usually be planted up to six weeks before the last frost date. It is probably best to wait a week or two after the last frost date to plant the warm annuals unless you can provide protection from frost. The average last frost date in Vancouver is mid-April. In fact we actually just had frost in most areas on April 15. So early May is a good time to plant warm flowers and vegetables. Some areas off the lowlands have later frost dates.
So how do we tell the cool annuals from the warm ones? For vegetables it is rather simple. If you eat the leaves, roots or flower buds, it is a cool vegetable with one exception: Peas are cool. If you eat the fruit, it is a warm vegetable with the exception of potatoes and sweet potatoes, which are warm.
What about annual flowers? There are no easily defined categories. You just have to learn them or look them up.
Here are a few of the most popular cool or hardy annual flowers: alyssum, carnation, cosmos, daisy, dianthus, dusty miller, nasturtium, pansy, petunia, poppy, snapdragon, sweet pea, viola. Of these, alyssum, cosmos, nasturtium, poppy and sweet pea are easily started from seed planted outside.
Here are a few of the most popular warm or tender flowers: ageratum, aster, begonia, celosia, coleus, dahlia, geranium, impatiens, lobelia, marigold, portulaca, red salvia, verbena, vinca, zinnia. Of these, marigold, portulaca and zinnia are easily started from seed planted directly outside.
To sum up, now is a good time to plant almost anything.