I am often asked about the best time to plant a particular plant. I usually say something to the effect that most plants growing in containers can be planted almost any time of year, but ideally I would wait for the cooler part of the year, beginning with fall. Of course, there are usually exceptions to rules, and this one is no different. I’ll discuss some of these exceptions later on.
So why is fall such a good time to plant? It comes down to one simple concept: plants are under less heat stress during the cooler days of autumn compared to the hot days of summer. Let me make an analogy. On a sunny, hot summer day I often hear people saying that it’s too hot to work outside, or to even be outside, or that they just don’t have any energy and can’t get anything done because of the heat. Our thirst increases and we find ourselves drinking more water or other liquids. Plants basically react the same way to heat. In hot weather, they are so busy taking up enough water to carry on their basic processes that it’s more difficult to expend energy to expand their root systems and become established. During the hotter times of the year we are also forced to pay closer attention to the water needs of plants in order to just keep them alive. In other words, more water is needed when trying to establish plants during the heat of summer than during the fall when temperatures are lower. With our current drought, this just doesn’t make sense.
During the cooler days of autumn, as well as winter and spring, plants, just like people, require less water. Natural rainfall, also, supplies some of the water needed, further reducing the amount of water we need to give them. As the air temperature cools and the days get shorter, the rate of shoot and leaf growth in most plants slows down. The plant doesn’t have to expend as much energy to produce new top growth, so when a plant is put in the ground, it can devote a greater proportion of its energy to grow new roots and become established. Don’t be fooled by the seeming lack of new growth of many plants during the cooler months. Many people get nervous when they don’t see any activity and think something is wrong. Be patient. What you can’t see is all the new growth underground. The plant is busy sending out new roots so that when the day length increases and temperatures warm, the new and larger root system can support the flush of new top growth.
So which plants benefit from being planted in fall? Most trees, shrubs and other plants that come from climates similar to ours are good candidates. This includes the majority of the plants commonly used in our landscapes. Cool season annuals such as pansies, violas, poppies and snapdragons need to be planted when the weather cools down in fall and will not perform well if planted during the warm summer months. Cool season vegetables also fit into this category, and include broccoli, cauliflower, cabbage, lettuce, spinach, kale and peas, to name a few.
Many of our native plants also need to be planted when the weather is cool. They are adapted to our Mediterranean climate with hot, dry summers. They start growing during the fall months when the temperature cools and rain normally begins, so fall and early winter is the time to establish them. Romneya coulteri, commonly called Matilija poppy is a good example. People usually want to plant them when they see them in full bloom in late spring or early summer. This is not the time to plant them, however.
I mentioned at the beginning that there are exceptions to fall being the best time to plant. The main one concerns tropical plants. Tropicals are adapted to growing in regions where the temperature is warm or hot and plenty of water is available all year. Although many will grow well in areas that are cooler than they are used to at certain times of the year, such as southern California, most may be difficult to establish during the cool months. Good examples are hibiscus, plumeria and bougainvillea. Although citrus may be planted all year long, the ideal time is in spring when the soil and air temperatures begin to warm.
If you’ve been putting off that planting project for some time, now might be the ideal time to begin. Your new plants will thank you for it.