About This Website
A short note before I dive into some geographical information. This website is intended to provide information on the performance and attributes of the many trees that can be grown in Sacramento and surrounding areas. The information provided should be fairly applicable to any inland, low-elevation areas of Northern California. It may also be of some use in the broader Southwest for areas that are warm temperate to subtropical, though a tree that does well in Sacramento is not guaranteed to thrive in San Francisco or Phoenix.
Growing Trees in a Sea of Grass
Though today known for its magnificent urban forest, Sacramento was not always so. While areas near rivers were naturally forested, most of the region was once open savannah or prairie. Few tree species can survive here without human care, and many others may struggle even with expert attention. What challenges stand in the way of fulfilling our green, shady dreams?
DROUGHT
This is probably the first and most obvious problem. Trees, like all living things, need water to live. Sacramento receives about 18 inches of rain in the average year, mostly in winter. That precipitation can keep things fairly moist through spring, but soils from summer to fall are usually extremely dry. For more than half the year, evaporation exceeds precipitation, and plants without the ability to store or access supplemental water will die.
In arid climates, most trees, even those adapted to drought, will not survive their first few years of life except in unusually wet years. So, if we want to ensure our trees succeed, they will need to be irrigated consistently until roots are well established. And all but the most drought resistant trees will benefit from irrigation even at maturity.
Trees that need higher amounts of water can often be grown successfully in our area, if adequate irrigation is applied consistently. However, with the current water shortages, it is wise to plant trees that have at least some drought resistance. Ideally, trees that require little to no supplemental irrigation at maturity should be selected. However, many adaptations to arid climates, such as small size, brushy structure, thorns, or slow growth may be at odds with other aesthetic or functional goals of tree planting.
HEAT
Another feature of our climate that is hard to miss is summer heat. Peak summer temperatures often exceed 110 degrees, and even temperatures over 115 are now possible. Sensitive species from cooler climates will burn, especially if water is scarce. Trees exposed to afternoon sun or reflected heat from built surfaces will be particularly affected. Even if direct damage from heat is avoided, species adapted to cooler temperatures will often struggle, appearing sickly or scraggly. In these cases, the cause may be hard to pinpoint, but the truth is that these trees would never experience such heat in their native climates, and they lack strategies to deal with it. Some of our most beloved trees, transplants from the Northeast or Midwest or even from cooler parts of California, fall into this category.
With climate change projected to make our summers even hotter, trees should be selected that have a proven track record of enduring and even thriving in our summer heat. Even better, we can look to performance in hotter climates such as the San Joaquin Valley or the deserts of Southeastern California and Southern Arizona which our summers may somewhat resemble by the end of a tree's lifespan.
FLOODS
With our system of levees, reservoirs, and bypasses, it can be easy to forget that much of the Sacramento Valley was once subject to frequent seasonal flooding. While the arkstorm may someday overwhelm our infrastructure, today most areas are flooded infrequently enough that this factor is often ignored. However, there remains another legacy of the valley's diluvial past. Fine particles of clay, deposited gradually from a thousand floods gone by, fossilized in some areas into layers of hardpan. This clay, which could be mistaken for cement when dry, can take many hours or even days to re-hydrate. Combined with the flat topography, this means that even if there is no visible flooding, our soils can change from dry to saturated after a single storm or poorly timed pulse of irrigation.
Tree roots need oxygen, and so most won't grow in saturated soils. While they can survive dormant for a time, too much time in wet soil will eventually kill them. While many trees can tolerate some wet conditions during cool weather, hot and wet is a recipe for more rapid illness. Even worse, many pathogens thrive in hot, wet conditions.
With the exception of a few scattered pockets of sandy loam, or sloped areas in surrounding hills, trees in the our area will need to be able to survive wet conditions in winter. To succeed in irrigated landscapes, most will need to tolerate some summer wetness as well. The whiplash from wet to dry is perhaps the greatest challenge for many trees, as some desert trees may not tolerate such moisture, and many trees of wetter climates will not last through the dry periods. Trees that grow in the lowlands and river valleys of hot climates often do best, as these areas can also range from very wet to very dry.
COLD
It may seem strange to describe our climate as cold--California is known in the rest of the United States for its mild winters. But due to the incredible diversity of tropical trees, the cold might restrict our planting palette more than any other factor. Most of the above challenges can be alleviated by careful site selection or certain cultural practices, but if a tree can't survive at least light frost, it is going to have a hard time growing here.
Sacramento is in USDA hardiness zone 9B, which means we can expect temperatures as low as 25 degrees in some years. Frost occurs almost every year, and over a tree's lifespan, it will likely experience a number of hard freezes. On longer time scales, lower temperatures are possible, like the freeze of 1990, when temperatures dropped to 18 degrees.
However, as above, climate change plays a role here. In this case, we finally see some benefit. Old wisdom said avocados couldn't be grown here, but there are dozens in my neighborhood alone. Even bananas are starting to appear here and there. While it is good to consider which trees were damaged in past freezes and how badly, we may never experience an event like the 1990 freeze again. In fact, 2013 was the last time we experienced our historically "typical" winter temperatures in the mid-20's. My yard, in the center of the urban heat island and surrounded by heated buildings, has not seen frost in years. While it's probably still not wise to grow anything that can't survive at least the low 30's or high 20's, there are many trees that were marginal in the past that are now worth trialing. But remember that climate change does not guarantee anything--another epic freeze could be right around the corner. It just gets a little less likely each year.