|My new lavender|
As soon as the soil begins to thaw in the spring the root growth kicks in again. Plants put in the soil in the fall now have at least six months of growth advantage over plants put in in spring. The larger root system provides more vigorous growth of foliage and flowers.
When you plant in the spring, the soil may still be relatively cool. That will slow and may even stunt a plant's growth. When you plant in the fall, the soil is still warm from the summer and root growth happens faster.
When you plant in the spring, many pests are just waking up and looking for food as they attack your young, vulnerable plants. When you plant in the fall, many pests are dead, dormant, or at a different point in their life cycle and not as much of a threat.
When you plant in the spring, a sudden, unexpected weather change can devastate new plants. When you plant in the fall, weather changes aren't as critical because the plants have already handled a winter.
When you plant in the spring, you must maintain a steady regimen of water until the plant stabilizes and then maintain that watering through the summer because the soil dries out. When you plant in the fall, less water is required as the plant begins to go dormant and more water is available in the soil from winter storms.
When you plant in the spring, you pay the highest prices of the year at your local nursery or home improvement center. When you plant in the fall, you can often get plants at reduced prices or clearance prices. As a frugal gardener, this is one of my favorite reasons.
When you plant in the spring, you may be guessing at the best location for new plants in new garden beds. When you plant in the fall, you have a better understanding of which plants grow best in specific areas of your garden. If you had a plant do especially well, now is the time to add more. If something didn't work out, now's the time to replace it.
You should allow at least six weeks for the root growth to take hold, so plan your fall plantings for at least six weeks before your traditional hard frost. That doesn't mean a first frost. In areas like the Front Range of Colorado, we may get a light frost of 31 or 32 degrees F, followed by weeks of warm weather. That may kill some foliage but isn't deadly for most perennials and trees. A hard frost is when both the air and the soil are below freezing. That typically happens at 28 or 29 degrees F. That's the point that signals plants to start going dormant.
If a hard frost is predicted, followed by forecasts of warmer weather, you can protect plants with a blanket or plastic tarp. Lay the protection over the plants before the sun goes down. The warmth of the earth and soil will create a pocket of warm air under the cover and will help keep the ground from freezing. This may buy you a few extra days or even a few extra weeks of root growth. Remove the cover after the sun comes up the next day.
Take a look at your calendar and see if you have the remaining time. In our area, continual hard frosts usually start at the end of October. That still gives time to add to the garden. You might have more days than that and more options. Take advantage of it and get planting.