IRA FLATOW: This is Science Friday. I’m Ira Flatow. The early part of winter here in New York has been welcomely mild. Almost springlike on some days. And throughout December, you could have taken a walk in Central Park and have spotted red roses still in bloom, dandelion blossoms dotting the lawns.

Well this week, reality returned. We got a little bit of real winter. As other parts of the country know, when they’re under the snow– we’re getting some winter now– so it’s bye-bye begonias for now. But the cold weather doesn’t mean you need to drop your spade and abandon your plants.

We’re going to give you some way to exercise your green thumb all during the winter. There’s still a lot of puttering you can do, like pruning back the roses. Properly storing those bulbs you’ve dug up. I have my own little mini experiment, well, maybe we’ll talk about with those bulbs. And finding the right potting soil.

You know, this is interesting. Not all potting soil is created equal. And not all alike. And we’re going to actually give you a recipe for making your own potting soil. So if you’re trying to troubleshoot your Winter Garden and plants, you want some advice, give us a call, 844-724-8255, or tweet us, @scifri S-C-I-F-R-I.

Elizabeth Murphy is a soil scientist, author of “Building Soil, A Top Down-to-Earth Approach.” She’s based out in Ashland, Oregon. Welcome to Science Friday.

ELIZABETH MURPHY: Hi. Thanks for having me.

IRA FLATOW: You’re welcome Gerard Lordahl all is the director of Open Space Greening for GrowNYC. He joins us here in NYC.

GERARD LORDAHL: Nice to be here, Ira. Thanks.

IRA FLATOW: Welcome to Science Friday. Gerry, what are the basics of what we should be doing to keep our outdoor garden right now?

GERARD LORDAHL: Well, in the wintertime right now, people are protecting the plants that might be a little susceptible to winter damage. And when we’re talking about soil, you want to protect the soil. Now that we have a frozen surface of soil, mulching is really important. And lots of our community gardeners are composting,

And these are all things that can be done now. Protecting your plants through the winter, giving as much water to them as possible on those mild days, and then mulching, are the two key things.

IRA FLATOW: ‘Cause they’re still growing. They need that water over the winter.

GERARD LORDAHL: Right, well, the surface of the soil is freezing with these freezing temperatures. And that freezing and thawing is what causes plant root damage. So I can’t encourage your listeners’ enough to mulch as much as possible, now that that surface is frozen. The problem is, when it’s not frozen, rodents can live under that layer of mulch. So now that the soil is frozen, it’s OK.

IRA FLATOW: Yeah, I noticed my camellias. They do terrible during the winter because they’re not getting the water.

GERARD LORDAHL: Right

IRA FLATOW: It’s frozen. Get out there and water them.

GERARD LORDAHL: So those days when it is mild, above freezing, you can go out there and try to water as much as possible. You mentioned pruning anything that needs to be protected. Those of you who have fig trees. Cutting back the roses. These are things that need to be protected with burlap, or straw, or mulch.

IRA FLATOW: Right. Elizabeth, I know that January is start to grow your indoor seeds month. Or get the bulbs going in, those daffodils and things.

ELIZABETH MURPHY: That’s right, yeah.

IRA FLATOW: How do we make– I know I want to talk about making your own potting soil. How do you do that? Because just what is soil? I mean, what is the difference between soil and dirt?

ELIZABETH MURPHY: That’s a very good question. And some people would even argue that potting soil isn’t really soil, because it’s not in the ground. So it really is this living ecosystem that’s connected through all the microbes and fungi. So we have a lot of challenges when we’re making a soil to use above ground, like in a potting mix or in our potted plants.

And a lot of those challenges are that we still are able to provide the air, the water, and the nutrients for growing plants. Even in these artificial environments. So when we’re talking about starting seeds, we really want to make a potting mix that has really good drainage. And that’s something that’s essential. We need to use either sand or vermiculite to get that drainage. And we want to use material that holds a lot of water and releases some nutrients.

IRA FLATOW: Mm-hmm. And we have, actually, some recipes for that up on our website at sciencefriday.com/pottingsoil. One recipe doesn’t contain soil at all, I understand.

ELIZABETH MURPHY: That’s right.

IRA FLATOW: What’s in it?

ELIZABETH MURPHY: A lot of propagation. Yeah, for starting seeds, the more popular mixes don’t actually contain live soil at all. Because real soil is actually quite heavy. And we want a light porous material for starting those new seeds.

IRA FLATOW: Can I scoop up all that soil I dug up outside during the winter, and stick it in? Jerry is just–

GERARD LORDAHL: Yeah, no.

IRA FLATOW: –he’s going crazy on this.

GERARD LORDAHL: And Elizabeth knows that. In her book, she mentions in her book talking about cooking soil. And many community gardeners and backyard gardeners will buy or make their own sterilized soil. The problem is there’s a disease called damping off that could affect young seeds, and it could kill the seeds. And it could be frustrating for gardeners who are trying to start seeds if they’re not using sterilized soil, as Elizabeth knows.

So cooking your soil in an oven at 200 degrees for 20 minutes is a good thing. Or use sterilized soil so you don’t get these diseases attacking your seeds.

ELIZABETH MURPHY: That’s right, yeah. You can buy that kind of soil, too, at garden centers.

IRA FLATOW: Now I know that a lot of potting soils contain peat. That must be–

ELIZABETH MURPHY: That’s right.

IRA FLATOW: Is it dug up literally from peat bogs? And is that a bad thing?

ELIZABETH MURPHY: Yeah, that’s right. And that’s one of the things that I like to educate people about. Because a lot of our horticultural industry is based on mining peat. And we are mining it. It’s not necessarily a sustainable practice.

And peat bogs are really important for carbon sequestration, globally. And in various places they are mined at unsustainable rates. So actually, in England, I think there’s a government mandate to move away from peat in horticultural industries by 2020.

So there’s lots of other alternatives that we can use. And this is one of the benefits of making our homegrown potting mixes. The purpose of peat is to hold on to moisture, and to release that to plants. And also to make your substrate nice and fluffy.

But we can use things like coconut coir, which is a byproduct of the coconut fiber industry. Compost fulfills some similar functions. Sometimes it’s a little heavy. Or we could use even ground-up newspaper. In small quantities can replace the function of peat. So we have more sustainable alternatives.

IRA FLATOW: Gerard?

GERARD LORDAHL: And here in New York City, we have over 500 community gardens that compost. And at our green markets at GrowNYC, we’re one of the largest networks of green markets. New York City residents can bring their kitchen food waste. So if you just made a pot of soup, and you had the carrot tops and the onion peels, you could collect all that and bring it to over 50 of our green markets city wide.

And then we have seven partner organizations that convert that into compost. And as Elizabeth mentioned, that could be used in place of peat moss.

IRA FLATOW: I’m going to ask you a botanical question.

GERARD LORDAHL: Uh-oh. I’m ready.

IRA FLATOW: Something that I’ve worried about as a gardener. I’m an avid Gardener, and I think about this. And I look at different kinds of fertilizers, and I know what the numbers on the fertilizer mean. But when I go back to my biology 101 class, and I look under photosynthesis. And the formula for photosynthesis is–

GERARD LORDAHL: C6 H12 O6.

IRA FLATOW: Yes.

[CHUCKLE]

IRA FLATOW: That’s–

GERARD LORDAHL: Yields glucose, right?

IRA FLATOW: Yeah. Well, the thing is, to create that. You have oxygen, and carbon dioxide, in the presence of sunlight. Creates that carbohydrate. Where is fertilizer mentioned in that formula anywhere? There’s no fertilizer. Where does the nitrogen fit in and all that kind of stuff? We don’t need it. It doesn’t seem like you need it anywhere.

GERARD LORDAHL: Well, on the cellular level, I’m sure Elizabeth can answer this question a little better. But in order for the processes of plants to function properly, they need the macro and micro nutrients. The nitrogen, the potassium, the phosphorus, the copper.

And those are on a cellular level, as I understand it. Having chemical interactions to undergo the processes of respiration and photosynthesis that are necessary. And also to produce that fruit that we love. That tomato. If we don’t have those nutrients, we won’t get those kinds of things.

IRA FLATOW: I’d like to get in–

GERARD LORDAHL: If that answers your question.

IRA FLATOW: No.

GERARD LORDAHL: No, it doesn’t.

IRA FLATOW: No, I like to get into the weeds.

GERARD LORDAHL: You need the scientist, Elizabeth.

IRA FLATOW: Elizabeth, you would know what that– what is going on there?

ELIZABETH MURPHY: Yes. Most of our tissues are carbon based. But take our muscles in our bodies, for instance. There’s a lot of nitrogen that form the amino acids, that form the muscles. So it’s the same with plants. To formalize the tissues, nitrogen is really important. It’s basically responsible for plant growth.

Take phosphorus, for instance, which is N-P-K. Our second nutrient in those fertiliser mixes. And think about ATP. Right? The energy of the body. So phosphorous goes into making that ATP.

Take potassium, for instance. It’s a salt. And so as a salt, it’s responsible for stomatal regulation. So opening and closing the stoma of plants, that allow that carbon dioxide in, that allow photosynthesis to occur.

So if we think about this holistic functioning of the body, of our bodies, of the plant body. There’s a lot of complicated stuff that requires a lot of these different nutrients, these different chemicals, these different elements. So it’s kind of exciting.

IRA FLATOW: It’s interesting. I think that explains it. One of the things that I’ve been doing over the last few weeks, before the ground it’s frozen, is to pull up my dahlia bulbs and things like that. And I always, I’m actually conducting my own experiment that I don’t expect to work very well. But being the kind of guy, I like to try things out.

They always tell you to take the tubers out of the ground and put them in sawdust, or paper, or something like that. And put them in a cool spot for the winter.

GERARD LORDAHL: Cool, dry, and dark. That’s correct.

IRA FLATOW: Right. Now I know why, and somebody explained to me, that’s why your onions are sprouting in the refrigerator. It’s cool, dry, and dark–

GERARD LORDAHL: Moisture, yeah.

IRA FLATOW: –in there, right? Same with the garlic. Do I have to take it out of the soil? If I have it in a pot, can I just bring the whole pot and leave it in the garage? Do I have to take it out of the soil?

GERARD LORDAHL: My experience is that the soil tends to retain moisture. So that’s why they want you to take it out and put it in something that is not going to retain the moisture. Like sawdust, or dry peat moss if you have it available.

Right now what’s on probably your listeners’ minds are those daffodil bulbs that they haven’t planted. And can they put them in the ground and will they have success. Well, the window of opportunity was up until about Thanksgiving for daffodils. And that’s because they need that cool temperature, but not freezing, to develop a root system that will enable them to get through the winter. Daffodils can last a little longer with that into Thanksgiving. Tulips as well.

So the refrigerator is a place where people could replicate that cold chilling temperature without freezing. If you put your daffodils that you haven’t had a chance to plant in the refrigerator, in a brown paper bag, dark, and dry for about six weeks or so. You’ll replicate that outdoor temperature and you’ll be able to force those daffodils indoors. You could then re-pot them.

IRA FLATOW: They’ll actually sprout? You’ll see them sprouting–

GERARD LORDAHL: Yes. I’ve had success doing it myself. Or if it gets mild, you can try and put those daffodils out in the ground at this point, if we have a mild spell. But it seems like we’re going towards the freezing temperatures these days.

IRA FLATOW: Wow.

ELIZABETH MURPHY: Yeah. And getting back to your question. Can I just take the soil and put it inside? Gets back to the question about– can I just use any old soil in my potting mixes?

So there’s a lot of different types of soil. So some soils are a lot heavier, have a lot of clays, and they’re going to hold on to moisture more. If you have a really light, sandy soil, you’re more easily going to get away with just bringing your bulbs inside in that soil, because they won’t stay wet through the winter as much.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from PRI, Public Radio International. Talking about gardening tips with Gerard Lordahl and Elizabeth Murphy.

So maybe I’ll have some luck. I’m trying to, literally, maybe I’ll have some luck. And being in the cool they’ll start sprouting up.

If they were to start sprouting themselves, what month what I expect to start seeing them?

GERARD LORDAHL: Are you talking about your Dahlias?

IRA FLATOW: Yeah. Well, I’m talking about– yeah.

GERARD LORDAHL: It depends. The different tubers and corms, like you have your gladiola corms, you have your dahlias. But the most successful are paperwhite narcissus that people force indoors. Or daffodils.

But gardening is all about timing. And things flower, and come into growth periods at different times of the year. You would never want to start your tomato seeds now. Amaryllis bulbs need to go through a dormant period in order for them to bloom. So there’s a lot of timing that’s involved.

And related to soil, a lot of listeners right now might be concerned in having their soil tested for correct pH. And that’s a big thing we do with the community gardeners in the wintertime. Make sure they have the correct pH. And as many scientists and gardeners know, if your pH is correct, whatever nutrients are in the soil will be made more available for the plants to benefit from.

IRA FLATOW: And if you have acid rain, it’s going to make the soil acidic.

GERARD LORDAHL: Well yeah. All those different elements can have an effect on the soil. Putting down lime increases the pH range of your soil. Adding sulfur decreases it. Blueberries, if you’re growing blueberries, they like an acid soil. So these are things to keep in mind.

But here in New York City, where we have issues with our pH because we’re surrounded by a lot of concrete. We encourage home gardeners and community gardeners to have their soil tested with simple pH test kits. And then adjust it accordingly.

IRA FLATOW: Elizabeth, any last words of advice for the wintertime?

ELIZABETH MURPHY: Well, I agree that it’s really important to check your nutrient balance in your soils, because we want to be successful as gardeners. And if you’re missing something really important, it’s going to be really disappointing. So getting that check is good. And a lot of that, too, is starting in the fall. So it’s great that we’re talking about soils in the winter, because we don’t always think about them.

And a lot of that work, the work for the spring, actually starts in the fall. Because that’s when the microbes in the soil wake up, and they’re ready to eat. So feeding them in the fall, especially through mulching like we’ve been talking about, is really important. So that we can burst into spring.

IRA FLATOW: Got the whole microbiome going on in the soil, there. Thank you, Elizabeth.

ELIZABETH MURPHY: Thank you.

IRA FLATOW: Elizabeth Murphy, soil scientist, author of “Building Soil, A Down-to-Earth Approach.” And Gerard Lordahl is director of Open Space Greening for GrowNYC, right here in New York.

One last thing! Remember the news about the four new elements that fill in the line seven of the periodic table? That’s 1, 13, 15, 17, and 18. It’s a triumph of chemistry. But better, it’s an excuse for us to play a little bit of Tom Lehrer’s “The Elements” song, as sung by the Notre Dame glee club at the live Science Friday show there. Enjoy.

[MUSIC – TOM LEHRER, “THE ELEMENTS”]

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