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Using Science to Care for Your Christmas Tree


Up next, it's the most wonderful time of the year, so how about something to get us into the holiday spirit - the science of Christmas trees. That's right, we're picking up where we left off a couple of years ago at this time. We spoke to a Christmas tree researcher in Canada named Raj Lada and we learned something startling.

This was amazing when we learned it: If you want your Christmas tree to stay healthy, don't leave a fruit basket gift underneath it. It knocked me out. Here's how it sounded: If you have a large bowl of fruit, will it give off ethylene?

RAZ LADA: Yeah, of course, don't keep your fruits close to your trees.

FLATOW: Seriously, keep the fruit...

LADA: Honestly, yes.

FLATOW: Yeah, yeah, if you get a fruit basket for Christmas, don't put it under the...

LADA: Exactly...

FLATOW: Don't put it under the tree. Get the fruit basket away from under the tree.

LADA: Get away, yeah, that's correct, actually, Ira.

FLATOW: And there you have it. Did you know that? I didn't know that. And so we thought we would tell you some more interesting stuff about Christmas trees. This is not to say that you should, you know, opt for an artificial tree if you have a fruit basket.

So if you are going with a real tree, what advice can we offer you this year? How about knowing which tree you should get? Yeah, or if you already have your tree set up, how can you keep the tree healthy and your home relatively needle-free? Well, my next guest has some handy tips on how to make a Christmas tree last into the New Year.

Rick Bates is an associate professor of horticulture in the College of Agricultural Sciences at Penn State. He joins us from WPSU in University Park. Welcome to the program.

RICK BATES: Good afternoon, Ira, thanks for having me.

FLATOW: Is there a specific species that makes a good Christmas tree, one better than the others?

BATES: There are. If you look at kind of everything that's out there on the market, you're going to see spruce, you'll see pine, you'll see Douglas fir, and then what we call true fir. And the true fir tend to be kind of like the Cadillac of the Christmas tree species.

In the east, Fraser fir tends to be the species of choice. In the western U.S., a lot of people really like noble fir.

FLATOW: So it's a fir tree?

BATES: Yeah, it's a fir as opposed to a pine or a spruce. Now...

FLATOW: How do you - do you know how to ask to see, you know, if you're getting the real fir tree or not?

BATES: Well, usually a retailer, even somebody at a big box store, for instance, usually could tell you the difference. Certainly the true fir, the needles tend to be soft. You can run your hand over a branch, and they'll be nice and flexible and soft. When we think of Christmas tree aroma, we're often thinking about firs, whether we know it or not. Balsam fir or Concolor fir tend to be really aromatic.

FLATOW: Right.

BATES: The spruce tend to have a - kind of a spinier, prickly needle. So if you grab hold of, let's say, a Colorado spruce, you're going to know it. The pines - for instance, white pine - tends to have a real thin, long, fine needle, real soft, real fine texture.

FLATOW: 1-800-989-8255 is our number if you'd like to ask anything about Christmas trees. You know, there's a lot of research. When we were researching this, Rick, we found that there's a whole bunch of research in Christmas trees, a lot of it going on. Why is that?

BATES: Well, there is. It's a big industry. If you look at how many Christmas trees we consume as a nation - the U.S. uses about 30 million, take or - you know, plus or minus a year. Europe, Western Europe particularly, they're closer to 50 million. So it's a billion-dollar-a-year-plus industry.

There are a lot of Christmas tree farmers across the country, probably in the neighborhood of 15 to 20 thousand producers of Christmas trees. So it's a big industry.

FLATOW: We're going to take a break, and we come back, we'll talk more with Rick Bates, associate professor of horticulture in the College of Agricultural Sciences at Penn State. Our number, 1-800-989-8255. You can tweet us, yes, tweet us @scifri, at S-C-I-F-R-I. If you have questions about what's the best time to put up the tree, what days, how to water it, what about a tree you want to keep live, how to replant - we're going to get into all that stuff, 1-800-989-8255 is our number. You can also tweet us @scifri, S-C-I-F-R-I, or go to our website at sciencefriday.com. And also go on our Facebook page @scifri. So stay with us. We'll be right back after this break.


FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. Of course we'll keep you up to date on the multiple shootings at a school in Newtown, Connecticut, and all the details will be available to you at ALL THINGS CONSIDERED tonight on the news.

Right now we're talking about Christmas tree science with Rick Bates, associate professor of horticulture in the College of Agriculture Sciences at Penn State University. Our number, 1-800-989-8255. Rick, let's get into some of the dos and don'ts of a Christmas tree. For example, when is the best time to buy the tree?

BATES: Well, usually sometime after Thanksgiving is when the Christmas tree sale season starts to heat up. It kind of depends on what your estimation is of how long you want to leave the tree up. You know, I normally go out and get a tree the weekend after Thanksgiving, and I procrastinate in taking it down, and so it sometimes doesn't come down until after the first of the year. So that may be five or six weeks.

So you don't want to push the envelope, or you might end up with more needles on the floor. So, you know, generally four to six weeks ought to be the limit. So if you're, let's say, you know, choosing a species that maybe doesn't have the best needle retention, let's say a white pine or a Scotch pine, maybe you want to wait until the first of December, in that first week of December, or the second week of December.

You may not want to try to leave it on display for quite as long.

FLATOW: Let's talk about a live tree. We've had a number of tweets. Nathan Rider(ph), (unintelligible), they want to know, first: Should I dig up an X-mas tree to plant after the holidays, how can I make sure it lives, how do I keep the tree and replant it after Christmas? What are the dos and don'ts for a live tree?

BATES: Those are all good questions, and it tends to be a little bit more complicated, you know, than the face value of the question. We have a lot of trouble keeping trees, live trees alive when we transplant them back outside in the landscape after Christmas.

And probably the best way to go about it, rather than actually digging up a tree and hauling it inside, would be to go to a local garden center or some of the retailers that are now offering this kind of product, like a containerized conifer, maybe a little bit smaller than a six- to seven-foot tree, but they're out there, and they're becoming more common.

Start with that kind of product, and then rather than planting it outside directly after the display period, after, you know, the first of the year, keeping it inside, something like an unheated garage. The light doesn't matter so much as just not exposing it to the real cold temperatures, especially here in the Northeast, after you've had it on display. And then...

FLATOW: Do you water it? If you have it in your garage, do you water the root ball at all, or...

BATES: You know, because it's a cooler environment, and it's not exposed to the wind, it doesn't use a lot of water. So rather than maybe a once-a-week watering, just check it frequently, maybe once a week, but I bet you only have to water it once every couple of weeks. It's not going to use a lot of water.

And then just as soon as the ground becomes workable in the spring, that might be March for some parts of the country, maybe April, that's when you want to go out and actually plant it. If you do that, I think your chances of that tree surviving in the landscape are going to be a lot better than if you just dig something up from a farm or from your yard, drag it inside and then replant it.

The other thing to keep in mind is that the longer you keep that kind of live tree inside on display in a warm living room, the worse your chances are going to be for that plant living once you plant it back outside. So you want to kind of limit the amount of time to maybe a week to 10 days inside on display versus a cut Christmas tree, you know, that's going to be recycled after the display period, anyway, so you can leave it inside until it actually starts to shed needles.

But for this live product that you want back out in your yard, you know, try to limit the amount of time that you're going to have it inside.

FLATOW: That's some shock it goes through if it's going to go from the warm to the cold.

BATES: Yeah, exactly.

FLATOW: Yeah, let's go to Bonnie(ph) in North Little Rock, Arkansas. Hi Bonnie.

BONNIE: Hi, I'm just wondering about what variety of tree is least likely to cause allergy problems. I've had some trouble over the past three years with when the tree comes in the house, and I'm thinking about an artificial tree, it would be the first one I've ever had. So I love a live tree.

BATES: That's a good question, and I hope you kind of stick with it and continue using a live tree. I don't know that there are really big differences between species in terms of allergies that we know about. There are some environmental factors that I think can lend, you know, to a tree maybe having the spores or the mold that might cause a problem.

If you have a period of time when the tree's been harvested that it's wet or particularly if it's warm and moist, and there in Arkansas you may have some November temperatures and conditions where it's relatively warm, there's a lot of moisture around, often when a Christmas tree producer or retailer has a cut tree, they will bail it, which means they'll wrap it in twine, bundle it up so that it stores better.

That kind of traps some of that moisture inside. So something like opening that tree up before you bring it inside, especially if it's been on display at a retail outlet under those kind of warm, moist conditions, that can help. Let it dry out.

FLATOW: Should you cut - put a fresh cut where the old one is?

BATES: Well, that's going to help in terms of the longevity and the quality of the tree inside. It may not do much in terms of allergies or spores. The other thing is a lot of choosing cut farms and the better retailers, they're probably going to shake the tree before you buy it. So they'll put it basically on a device that simply vibrates the tree, and that knocks out any kind of loose needles or dead needles that may be up in the tree.

And that gets rid of a lot of the debris, a lot of the structures like the dead needles, where, you know, there might be a spore or something on that needle. And that can kind of clean the tree up. So that can help. Most conditions, people generally don't have much of a reaction.

FLATOW: Bonnie, thanks for calling. Good luck to you.

BONNIE: OK, thank you.

FLATOW: 1-800-989-8255. Let's go to Stan(ph) in Tallahassee. Hi, Stan.

STAN: Hey, yeah, I had a question. We have a - one year we let our tree stay out on the back lanai, and we're in North Florida, and the temperature was cool, and there was a lot of sun. And I was wondering: Does the - does that affect - how does temperature and light affect how long your tree will live? And I'll take it off the air.

FLATOW: OK, thanks. Yeah, what affects how long your tree with live? And maybe you can throw in that question I asked about cutting it when you get it home. Does that affect how long it lives?

BATES: It does. The best way to look at it, I think, Ira, is it's a combination of genetics and environment. And the genetics are based in the tree. So there'll be some tree-to-tree differences and some differences between these species that we've talked about. But then the other big factor are the environmental conditions that the tree is exposed to.

And really what we're talking about are the environmental factors that relate to water, to hydration of the tree and to how quickly the environmental factors around that tree pull water out of the tree. And so the key is if you want to control the environment is to try to keep the tree as hydrated as you can during the entire display period.

And so that would start with, like you mentioned, fresh half-inch or inch cut on the base of the tree if you don't know how long it's been since the tree was harvested. That's going to open up the xylem vessels to take in water. It's going to open the tree up to absorb water. And trees actually can drink a lot of water, especially when they're freshly cut.

So it's not uncommon for a tree to absorb a gallon or more a day.

FLATOW: A gallon a day?

BATES: A gallon a day. A good rule of thumb is about a quart of water per inch of stem diameter per day. And this goes down kind of over the lifespan of the tress when it's inside. But initially, that could be a lot of water. And then...

FLATOW: What about those plant packets they put inside? Do those help keep the tree alive?

BATES: The research that we have to date don't really indicate that there's much benefit from that. You'd be better off concentrating your efforts on things like making sure you've got a proper reservoir of water, something that holds at least a gallon, keeping freshwater in there.

Like your caller suggests, maybe keeping it out of the sun, out of drafts, away from heat, a heater vent, or something like that can tend to dry the tree out. So what you're really trying to do are modify or reduce all those factors that would suck moisture out of the tree, and that's going to probably do more than anything else to extend the life of the tree on display.

FLATOW: One question for you about, you know, we're looking forward - what's the best way to dispose of your tree once the holiday's over?

BATES: Well, when the holidays are over and, you know, I don't know if you're like me, I tend to push the envelop and I don't really take the tree outside until I kind of start to see needles dropping and it clearly is on its way out. Most municipalities now have a good recycling program where they will turn that Christmas tree into mulch, just like they would branches or leaves in the autumn. So they'll take the tree, and they'll put it into that composting waste stream and turn it into something useful that the municipality then often either gives away or sells in the form of mulch. So it's a great way to recycle. And actually, if you get on the website realchristmastrees.org, you often can actually find a recycling program in your municipality.

FLATOW: I heard that you have a novel or have come up with a novel method of disposal of Christmas tree that we wouldn't have thought of or expected - in a lake. Is that correct?

BATES: Yeah. If you're out in the country or around a reservoir or a farm pond or farm lake, some people will take the Christmas tree and actually toss it into the lake, and it actually provides some habitat for small fish.

FLATOW: Will they allow you to do that?

BATES: Oh, you probably want to ask first. I - a number of years ago lived on - next to a farm and - that had a couple of farm ponds, and the farmer was happy to have the Christmas trees in the lake because there was a pretty healthy bass population and bluegill population. So it's a good practice. Not everybody can do that if you're in an urban area, but certainly that or recycling the tree is a real good solution. It really is a renewable resource, and so we grow the trees and turning them into mulch after the season is a good way to dispose of them.

FLATOW: And what about the temperature of the water? You say we need a gallon jug to put it in, to keep it filled with a gallon. Does it help to make it hotter water than, you know, than average temperature?

BATES: It really doesn't matter. Cold water is fine. What does matter is the fresh cut on the base of the tree. If it's been, you know, a day or so since that tree has been harvested, putting the fresh cut opens up those pores that are going to draw in the water. And if the water is cold or hot, it doesn't matter so much as having a fresh cut and no sap covering that base of the trunk.

FLATOW: So, that's the most important thing then? Enough water...

BATES: Yeah.

FLATOW: ...give it a fresh cut and have a Merry Christmas.


BATES: Yeah. And enjoying the tree, and, you know, those things like keeping it maybe out of the sunlight...

FLATOW: Out of the sun.

BATES: ...out of drafts, away from heater vents. But then checking the reservoir routinely because I find that commonly people are surprised just with how much water the trees, especially in that first week, so keeping an eye on that Christmas tree stand reservoir.

FLATOW: And keep the fruit baskets away from it. Thank you very much, Rick.

BATES: OK. Thank you.

FLATOW: Happy holiday to you. Rick Bates, associate professor of horticulture at Penn State University. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR. Transcript provided by NPR, Copyright NPR.

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