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Plant Answer Line Question

Keywords: Lathyrus odoratus, Plant phenology, Pisum, Planting time, Syringa

It's already the middle of March and I'm worried that our soil is still too cold to plant peas (both edible and sweet). When is the correct time to plant them in the Seattle area?


Since weather patterns vary from year to year, it may make more sense to plant based on something other than the calendar date. An old adage says that it is time to plant peas when the lilac leaves are the size of a mouse's ear. This may sound quaint, but it turns out that the growth cycle of the lilac (Syringa) is an excellent indicator of temperature. Phenology is the science concerned with the timing of specific biological events, and lilac is among the plants often studied. Project BudBurst has additional information about phenology and climate change. The U.S. National Phenology Network is also a good resource.

If you don't have a lilac in your garden (or a mouse's ear, for that matter), Washington State University Extension says that a safe time for planting peas is usually mid-March, not so much because of soil temperature, but because in February the soil is often oversaturated, and your peas would rot in the ground.

Date 2017-08-24
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Plant Answer Line Question

Keywords: Plant phenology, Leaves, Trees

Can you direct me to a list of deciduous trees whose leaves generally emerge in early spring, or a list of trees ranked in order of their leaf emergence? I know this will vary from year to year and from individual tree to individual tree due to climate and genes, but if there is a list out there with a general sequenced time schedule, it would be a great tool for design.


An early American observer of the varying timing of leaf emergence was Henry David Thoreau, whose journals list leaf-out dates for the trees and shrubs he saw in Massachusetts in 1854. In fact, his data is now being used in climate change research. Though it's a subject that hasn't been much approached from a garden design standpoint, the increased interest in climate change means that more research on phenology and the leafing out sequence is becoming available. See the following article, "Turning Over New Leaves" by Richard Primack in the Arnold Arboretum newsletter. Primack (of Boston University) is the author of many articles on this topic. Primack and Caroline Polgar co-authored "Leaf-out phenology of temperate woody plants: from trees to ecosystems" (New Phytologist, Volume 191, Issue 4, pages 926-941, September 2011) which states that "maples (Acer spp.), birches, alders (Alnus spp.), and poplars" tend to leaf out earlier, while "oaks, ashes (Fraxinus spp.), and hickories (Carya spp.)" are among the later-leafing trees.

The article "Why Do Temperate Deciduous Trees Leaf Out at Different Times? Adaptation and Ecology of Forest Communities," (The American Naturalist December 1984, Martin J. Lechowicz) has a chart (p. 825) showing the tree species the author studied leafing out in this order:

  • Acer rubrum
  • Populus tremuloides
  • Betula papyrifera
  • Sorbus americana
  • Acer saccharinum
  • Betula alleghaniensis
  • Ulmus americana
  • Tilia americana
  • Quercus macrocarpa
  • Fraxinus pennsylvanica
  • Populus grandidentata
  • Fraxinus nigra

Another chart on the same page compares 1980 and 1981 leafout dates, with Populus tremuloides and P. balsamifera and Betula species consistently leafing early, followed by Acer and Prunus, then Fagus and Populus grandidentata, then Fraxinus and Tilia, and finally Carya and Juglans.

You may want to read a short article in the March 11, 2015 online version of Conservation Magazine on predicting the future of forests, based on two centuries of data from citizen scientists in England. Here is an excerpt:
"It is likely that the variation in each species' sensitivities to both spring forcing and winter chilling will mean that forests will look quite different in the future. Those species for whom spring forcing is most important will grow leaves earlier in the year; those for whom the autumn and winter chill is more critical could leaf later in the year. Eventually, a late-leafing species like oak might wind up growing its leaves earlier than an early-leafing species like birch."

Another resource that may help you determine the leaf-out date of specific trees is The Botanical Garden: volume 1: Trees & Shrubs by Roger Phillips and Martyn Rix (Firefly, 2002). The book presents photos of branch samples from many tree species, often showing the young leaves associated with a date (though not with the geographical location; bear in mind that the authors reside in England). While it doesn't have such a photo for every tree, it might have enough trees for you to get useful data.

Date 2017-01-12
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August 01 2017 12:36:01