Deciduous fruit trees need plenty of sun, water, and nutrients to produce good fruit but they also needs plenty of cold weather. A period of dormancy, rest, is required to produce good fruit. When a tree is dormant, it needs to accumulate a certain number of chill hours, hours between 32-45 F, before resuming growth. This is an adaptive strategy to avoid bursting into bloom during a warm week in midwinter and to synchronize emergence with the pollinators. 

Notes on how chill hours are measured

  • Trees accumulate chill hours most effectively under consistent cold temperatures.
  • If daytime temperatures exceed 60°F, previously accumulated chill hours may be lost.
  • Sub-freezing temperatures (<30°F) do not contribute to chill hour accumulation.
  • You can look up chill hour accumulation for a specific location here. 
Abscisic acid (ABA) is a hormone that develops in the tips of branches, suppressing growth until ABA is worn down. The amount of ABA produced and chill hours required is directly correlated.

Many fruit trees that we grow come from northern temperate regions of the world, with cold winters. Dormancy became the survival mechanism, chill hours the adaptation. The number of chill hours required stems from the trees native climate, how it adapted to survive there. In the Mid-Atlantic region, we typically get about 1,000-1,500 chill hours each winter. Fruit trees will be healthiest growing with similar chill hours and climate as their native lands.  Trying to grow fruits with widely different chill hour requirements can prove challenging. 

Not enough chill hours

I witnessed the effect of not enough chill hours in the orchards at the UC Santa Cruz Farm and Garden in 2015. It was an unusually warm and dry winter, and chill hours were low. Instead of a synchronized burst of energy and color, the apples, cherries, and peaches all woke up staggered and unenthusiastic, or not at all. One branch would be setting fruit, while another hadn’t yet flowered – like deep into the summer. The orchard being out of rhythm made it harder to manage logistically and the yield was low. If the chill hour shortages persist for multiple years, the trees may weaken and die. And there is not much we can do about it. The tree is out of balance because the ecosystem is out of balance. 

As we continue to trend towards generally milder winters, it may be necessary to adjust plant and variety selection in the region to better match the reduced chill hours we are receiving. For many common fruits, there are varieties that require fewer chill hours that have traditionally been considered better adapted to the southern United States. Philadelphia is effectively migrating South through the effects of climate change.

Fruit trees at Grumblethorpe bursting into color in unison.

Too many chill hours

Breaking dormancy is influenced by both achieving required chill hours and by temperature and seasonal light changes. Although a fruit tree doesn’t automatically break dormancy once it has reached its chill hour requirement, it does become susceptible to breaking dormancy early with the onset of warmer temperatures. For this reason apricots and Japanese plums (especially Santa Rosa and Satsuma), which only need 300 chill hours, have not been reliable crops in Philadelphia. In many years, they are blooming too early, getting hit by a hard frost, and losing their crop for the entire year. Sometimes a few trees throughout the city will have survived the cold temperatures due to being tucked in a warm microclimate – often those near pavement and buildings, the south side being especially warm. With climate change, the weather is going to become more sporadic and disorienting to the trees, and we will likely see this happening more and possibly to other fruits besides apricots and plums. 

Partners at Wyck successfully protected branch of apricot flowers from a hard frost by wrapping.

Protecting blossoms from frost damage

When temperatures dip down to 27 F, frost damage hits hard – critically damaging blossoms, young leaves, and fruit. But there are a few hail mary’s you can try.

  1. Cover trees. Wrap individual limbs with row cover or cover top of tree with double layer of bed sheet and then plastic. Place buckets of water under the canopy. 
  2. Run overhead irrigation high in canopy starting before it drops below freezing (for sake of the hose) and continuing until temperatures rise. When water changes phase from liquid to ice, it actually releases heat!
  3. Hang outdoor rated incandescent christmas lights in the tree. LED lights do not provide heat. 

Check out this comprehensive article How to Protect Fruit Trees from Frost by Susan Poisner of Orchard People. Includes everything you need to know to give protecting trees from hard frosts a shot. 

Paw paw flowering at the Awbury Food Forest. Flowers emerge before leaves in spring making them easy to find for pollinators.

Tree Phenology: Dormancy and Chill Hours

1. Acclimation (Preparation for Dormancy)

  • Begins in midsummer when shoot extension stops, and terminal buds form.
  • Before shedding leaves, trees reabsorb nutrients, especially nitrogen.
  • Bud scales develop, appearing swollen and fuzzy—these structures wrap around buds to protect them from winter damage.
  • The plant produces abscisic acid (ABA), a hormone that signals dormancy and prevents premature growth. Trees with higher needed chill hours produce more ABA.

2. Endo-dormancy (Deep Dormancy)

  • During this phase, trees accumulate chill hours—a necessary cold exposure that ensures proper bud development.
  • ABA, stored in the bud scales, prevents premature growth even if temperatures briefly rise.  ABA is located in bud scales and sinks with gravity (just like auxin) down to the rest of the tree whispering, “Don’t grow yet, wait for it.” It’s protective, signaling to hold steady during a warm snap mid-winter.
  • Flower buds are developing. Flower buds are the first to reactivate in spring, are tightly regulated to optimize pollination timing.

3. Eco-dormancy (Waiting for Warmer Conditions)

  • Once the required chill hours are met, trees remain dormant due to ongoing cold temperatures.
  • As temperatures gradually rise, sunlight and warmth degrade ABA, allowing bud growth to resume.  

4. Deacclimation (Breaking Dormancy)

  • When temperatures reach 40°F, trees begin accumulating degree days instead of chill hours.
  • This phase regulates bud break, leaf expansion, and flowering.

Final Thoughts

Climate change is changing our ecosystem in ways that seem to get louder every year. Winters are trending warmer and drier. Weather is sporadic. We are feeling the increase in frequency, intensity, and duration of extreme weather patterns. And the trees are feeling it too. Witness it. Adapt to it. Adjust plant and variety selection for the changing climate. Be inspired by their resilience, understand their needs, protect them when you can. 

Sources: 

Click to access Chill-Hours-and-Identifying-Fruiting-Wood-Presentation.pdf

https://extension.msstate.edu/publications/chilling-hour-requirements-fruit-crops

https://extension.psu.edu/what-causes-a-tree-to-enter-and-exit-dormancy

https://www.davewilson.com/home-garden/fruit-variety-recommendations/fruit-tree-chilling-requirement

Fruit Trees for Every Garden – Orin Martin 2019

Botany for Gardeners, Revised Edition. Brain Capon. 2005. 

How to Protect Fruit Trees from Frost – 3 ways – Orchard People 

This POP Blog was written by Education Director Corrie Spellman-Lopez.

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