Why Do Trees Leaf Out?

Mar 23, 2018

This is a question that fascinates me. When I look around in spring, I notice that although the maples and the magnolias are out, the hickories and sweet gums are not. Why is that? Today I’d like to take a shot at answering that question.

I’ve figured out there seem to be three major factors that influence when trees emerge from dormancy:

The scene in my backyard right now - pure emerald - as the trees leaf out.
The scene in my backyard right now – pure emerald – as the trees leaf out.

The first is temperature. I know this may seem odd, but autumn plays an important role in when the trees break dormancy in spring.

Warmer autumns cause the trees to leaf out later, while cooler autumns induce the trees to start growing early. I found this hard to believe, because that would mean that our winters are almost the same lengths every year, but evidently they are. It’s just that they start and end at different times, but (give or take a week) generally last as long.

This is where chill hours (which are the number of hours under 45 degrees that a tree needs to break dormancy) come into play. I often hear that term associated with orchard trees, but it applies to every tree. Each tree has a set number of hours in its genetic code that prompts it to grow after the amount of chill hours have been reached.

Since some of the trees around you are imported from other countries, they have different requirements than those native to your area. Even the trees in your area vary for one reason or another. Some leaf out early (like the magnolias) and risk being caught by a late frost to get a head start on the competition. Others prefer to wait until there is absolutely no chance of frost before they attempt to grow.

Of course, there are always those crazy years when even the later group gets caught by a frost, but that is rare. Keep in mind that this is nature we’re dealing with. Although it may not make sense to us, the trees are nature, so they understand perfectly.

 

The maple in my front yard responds to heat - when the temperature reaches 60 degrees (even in February) it's ready to grow!
The maple in my front yard responds to heat – when the temperature reaches 60 degrees (even in February) it’s ready to grow!

The second is light. Some trees (especially in tropical areas) use light as their cue to wake up. Since light almost never varies each year, it is an extremely reliable way of determining the seasons, as long as there’s not frost in the equation.

This is how growers induce Poinsettias, which go dormant in the summer, to flower at the right time. They simply restrict the light supply, and the plants respond by flowering.

The last factor is water. Since all ecosystems have some sort of weather cycle, the trees use it as a signal to break dormancy.

For instance, where I live, there is almost always a dry spell in the late summer and autumn. This prompts the trees to start changing colors and drop their leaves. The only tree in my yard that does not readily respond to this signal is the Tree Of Heaven (Ailanthus altissima), since it is nonnative and has different ways of determining when to go dormant. The same principle applies to the spring. When the rainfall decreases (combined with other factors such as temperature and light), the trees break dormancy.

I hope this has shed some light on such an interesting (and rarely talked about) topic!

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  1. Brenda Hart Neihouse says:

    🙂 thanks for the reminder as we suffer through a few weeks of intermittent Frozen nights and warm days. I’m worrying about everything that was starting to Leaf out. We to have a hickory and we’ll have to have discussion sometime because it’s new to me as of 2 years ago. I’m hoping to get out today and do some seeding of Hardy annuals and perennials and take a good look at everything that’s starting to Leaf out.

    • Travis says:

      Cool! There is still a tiny chance of frost here, but since the average last frost date is only a week away, I’d say Louisiana is most likely in the clear until this next fall! 🙂

      It always gives me hope when – after many dreary winter days – I can finally step outside and see green once again. It’s one of the best feelings!

  2. Kathleen Askins says:

    The photos of your trees are just stunning! Thanks for sharing! Here in PA, where it just snowed again this morning, I am happy to see the pretty spring green in your pics!

  3. Paul Manhart says:

    Still the depth of winter here in se Michigan, lows of 19 and 22F later this coming week..Tulips and Daffodils just barely peeking out of the ground and really no signs of anything leafing out any time soon!

    • Travis says:

      It’ll happen before you know it! One day – nothing. The next – they’re in full bloom! 😉

      Hopefully spring will head your way shortly!

  4. Lauren🐝 says:

    Say, what sort of maple is that in your front yard? Also, what an interesting explanation. I figured on 1 & 2 but never considered 3. Thanks very much. Best of luck in your endeavor. Uch we got 5″ of snow this morning. BUT it has all melted so we are back to just mud!

    • Travis says:

      Hi Lauren!

      That is a Drummond’s Red Maple (Acer rubrum var. drummondii). They perform well in zones 5b-9 and even show color in our hot autumns – it’s one of my favorite maples!

  5. JoyceBinAtlanta says:

    North Atlanta (zone 7) is in full spring right now. The Autumn Azaleas I planted about 8 years ago look better than ever. Last year one of the varieties stayed in bloom from spring through fall. It’s interesting to be reminded that non-natives sometimes don’t follow our conditions. I’m so spoiled with the ease of growing Japanese Maples here, that I didn’t think about why other plants seem dead before they suddenly pop up. We had a tough winter this year with record cold – we even had snow! Though the plants seem to have survived, I do hope some of the bugs didn’t!

  6. Liz Kemp says:

    Thanks for the very interesting article, Travis – especially about the consistent length of winter; I’d never heard that before. I’m in the same state of muck as Lauren, but sure enjoyed the snow while it lasted. I did a little raking over the weekend (lots of leaves due to having taken the “lazy gardener” pledge in the fall) and am hoping that all the little plantlets that I uncovered can hang in there while we warm up again.

    Great start to the newsletter!

    • Travis says:

      Thanks, Liz!

      I took the “lazy gardener” pledge as well – It’s been hard not to clear out all the debris – but it was worth it! I’ve already noticed an increase of wildlife in the garden. Birds, frogs, and butterflies have overwintered in the leaves – it’s amazing! 🙂

  7. Rebecca davis says:

    Ha! I had that snow, too, Sunday night and I love snow! It looked beautiful and I remembered the old adage that snow at this time of the year is a “poor man’s fertilizer”. I kept telling myself spring WILL be here soon and the snow was watering the English lavender and “Autumn Brilliance” serviceberry I just planted. It was good to read the other conversations and feel comraderie with all you gardeners in the same boat. It is getting darker as the morning goes on with predictions of rain so no outside work today.
    I would like to give the overall reason why things grow as they do! We have an amazing Creator who has place the genetic code in each and every little growing thing from our tiniest insect to the most gigantic tree. As we study the complexity of nature He is the only logical reason though not the easiest to understand!
    Can’t wait until my honey bees are back! I lost 6 out of 7 hives this past winter aaarrrggghhh

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