The USDA plant hardiness zones provide a very important piece of information to gardeners – how cold can the weather get without killing my plant. But this only touches on the tip of the iceberg. For many plants, even the USDA zone information needs to be modified by how wet it is at the time of the cold. For example, many plants will survive cold so long as they are relatively dry. Give them cold and wet and they will die.
Some plants will also provide an upper range in which plants can be grown. It is not uncommon to see a plant specified as zones 5 to 9. This is not really very useful at all. It is trying to convey that some plants cannot take excessive heat, or humidity or both. But hardiness zones and heat are not correlated for much of the country.
The biggest exception is the entirety of the West Coast (California through to British Columbia) where the Pacific Ocean has a huge influence on both the highs and the lows. The ocean provides an enormous stability. In winter we don’t get very cold, and in summer we don’t get very hot. Go inland just a few miles and things can be very different.
The previous paragraph contains an innocuous statement ‘in summer we don’t get very hot.’ That is one of the biggest impediments to what I can successfully grow here at Oceanside Garden. Many plants will remain dormant until the soil or the air have warmed up enough to spur them into growth. There is one plant I was trying to grow – Bauhinia purpurea, the purple orchid tree. It is a delight and would have been perfect except for one small problem.
Last year, the Bauhinia didn’t even break bud until August. It sent out one new shoot that grew to be three inches long and had seven leaves on it. When talking about a tree and with that kind of growth rate it is going to take a very long time to reach a decent stature. Plus, it is unlikely that it would ever set flower buds or that they would ever get to open before cold weather came in.
Unfortunately, that tree will be going on the compost heap in spring and a replacement found.
Death by Heat
To deal with this issue, a number of alternatives have been suggested. One comes from the American Horticultural Society that defines heat zones. They divide the country up into 12 zones based on the number of days temperatures rise above 86F (30C). For us that is almost never and thus we are zone 1. Zone 12 is for areas where this happens more than 210 days a year. Again, this is not useful because it defines when things will die and being in Zone 1, that means that nothing will ever die from heat here.
We have similar problems with tomatoes, which grow so slowly here that they often do not ripen. We need something that tells us about the temperature below which the plant will not fully function. For example, I have seen orchids often being described as sulking below 50F. Thankfully, I have a little space available in the greenhouse over summer and so there is room for a plant or two. I have even successfully grown plants over winter and had ripened fruit in March.
First and Last
Another useful piece of information is the average first and last frost dates. This can help knowing the length of the growing season – at least for plants that are willing to grow at temperatures barely above freezing. But these are averages. If you live by those, you may not take advantage of all of the growing season. Most people tend to stretch a little and cross their fingers.
Sunset attempted to fix all of these issues, at least for the West Coast. They divided the region into zones that consider the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, wind, and humidity. This works for well-known plants that have been trialed in a lot of places, but growing members of the Protea family is still restricted to a few of us adventurous types. Let’s face it, nobody really knows much about how well they can grow in this country.
Lack of Data
The lack of heat is a concern for a number of the plants. There is little information on the Internet about the amount of heat they need to set flower buds. A few locations suggest they need temperatures higher than we get. So far, they appear to be wrong. A number of plants have bloomed, but I think the right question may be – do they bloom as well as they might in a warmer climate? I am OK with that. Part of the reason for wanting to try some of these plants is because we are in a warming climate and many native trees and shrubs are beginning to suffer.
This summer, the forests looked quite sad with a brown tinge over their normally bright green coats. It takes time for new trees to settle and become established. Just like many of the English gardeners – they planted for the future, not for today. Many of what are considered the great garden landscapes took 100 years to reach a modicum of maturity. We would not have had them if the gardeners of the time had not thought about planting for future generations and not for instant gratification.
Can I say I am planting a garden for the next hundred years? Perhaps not, but I do have a longer term view than most people.