346 All About Soil Thermometers, pH Testing, Cardboard Mulch

How do use and adjust a soil thermometer? Why is my soil pH so variable at different soil depths? How do you use cardboard to replace a lawn and start a garden?

Those are some of the questions, along with scenic bypasses - of course - that we’ll be tackling today. And there’s a lot of great garden tips in those scenic bypasses today.

It’s all in Episode 346 of today’s Garden Basics with Farmer Fred podcast! Brought to you today by Smart Pots and Dave Wilson Nursery.  Let’s go!

Previous episodes, show notes, links, product information, and transcripts at the home site for Garden Basics with Farmer Fred, GardenBasics.net. Audio, transcripts, and episode chapters also available at Buzzsprout.

Pictured: Soil Thermometer

Links:
Subscribe to the Beyond the Garden Basics Newsletter https://gardenbasics.substack.com
Smart Pots https://smartpots.com/fred/
Dave Wilson Nursery
HeirloomRoses.com (with the FRED discount link)

Other links mentioned in today’s podcast:
Soil Thermometers
Instructions for Soil Thermometers (Australia)
Soil Thermometer calibration (Wisconsin)
Soil Temperature Chart for Seeds
Sheet Mulching with Cardboard (UCANR)
What is Hugelkultur? (Oklahoma St. U)
Understanding pH (UCANR)
pH Test Kits

University Soil Testing Services: Texas A&M, Colorado State, UMass/Amherst

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Farmer Fred Garden Minute Videos on YouTube

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Chapters

• 1:17 Q&A Soil Thermometers
• 11:58 Smart Pots!
• 13:43 Q&A Soil pH variables
• 20:43 Dave Wilson Nursery
• 21:55 Q&A Using Cardboard to Kill a Lawn
• 27:35 Want to Leave Us a Question?

Transcript

346  TRANSCRIPT Thermometers,  pH, cardboard

Garden Basics with Farmer Fred is brought to you by Smart Pots, the original lightweight, long lasting fabric plant container. It's made in the USA. Visit SmartPots.com slash Fred for more information and a special discount, that's SmartPots.com/Fred.

Welcome to the Garden Basics with Farmer Fred podcast. If you're just a beginning gardener or you want good gardening information, you've come to the right spot.

Farmer Fred

How do use and adjust a soil thermometer? Why is my soil pH so variable at different soil depths? How do you use cardboard to replace a lawn and start a garden?

Those are some of the questions, along with scenic bypasses - of course - that we’ll be tackling today. And there’s a lot of great garden tips in those scenic bypasses today.

It’s all in Episode 346 of today’s Garden Basics with Farmer Fred podcast: All About Soil Thermometers, Soil pH, and cardboard mulch.

We’re podcasting from Barking Dog Studios here in the beautiful Abutilon Jungle in Suburban Purgatory,  it’s the Garden Basics with Farmer Fred podcast, brought to you today by Smart Pots and Dave Wilson Nursery. Let’s go!

Q&A HOW TO USE A SOIL THERMOMETER

Farmer Fred

How do you use and adjust a soil thermometer? Why is my soil pH so variable at different soil depths? How do you use cardboard to replace a lawn and start a garden? Those are just some of the questions along with scenic bypasses, of course, that we'll be tackling today. And there's a lot of great garden tips in those scenic bypasses as well. It's all in episode 346 of today's Garden Basics with Farmer Fred Podcast, all about soil thermometers, soil pH, and cardboard mulch. We're podcasting from Barking Dog Studios here in the beautiful abutilon jungle in suburban purgatory. It's the Garden Basics with Farmer Fred podcast, brought to you today by Smart Pots and Dave Wilson Nursery. Let's go. 

Farmer Fred

We like to answer your garden questions here on the Garden Basics podcast. Debbie Flower is here, America's favorite retired college horticultural professor.

We get a question from Curious Craig who says, 

“I’m curious about how and when do you measure soil temperature? Also, I really appreciate your content. You have the perfect blend of personal touch and meaty content.” 

Consider me the meat, and Debbie the personal touch. 

“Love the addition of a respected voice such as Debbie Flower who contributes concentrated advice and respected content. I've heard other garden podcasts that are pure fluff. Lots of talking about opinions and no facts. Keep up the awesome work.” 

Thank you, Craig. It's not easy. Back in the old days, all we had to deal with was Jerry Baker. Now it's with everybody else who’s online. 

Debbie Flower

Yeah, now it's magnified by the internet. 

Farmer Fred

So thank you. Yes, we try to be your trusted source for gardening information. Soil thermometers are widely available at many nurseries, but usually with that soil thermometer is very little information on how to use it or how to care for the device when in reality there's a lot that needs to be taken care of. It is a precise measuring device that needs to be taken care of. You just can't leave it sitting in the soil, for example. 

Debbie Flower

No you can't because it'll heat up from the sun. They're typically metal because you're putting them in wet soil so they heat up from the sun. You can't do that. 

Farmer Fred

So, what do you do? 

Debbie Flower

You put it in the soil, and let it sit.

Farmer Fred

 How deep do you put it in the soil? 

Debbie Flower

Good question.  Depends what you're measuring it for. 

Farmer Fred

Very good point. Proceed. 

Debbie Flower

If you're measuring it for seed, you're going to go very shallow, maybe an inch or two inches. Because that's where the seed is going to sit. And that's what you need the temperature for. If you're going for, let's say transplanting as annual, you want to put those zinnias out or you've started your cucumbers in pots, then you're going to go about six inches deep. You need to leave it there. Mine, I don't know about yours, mine goes very slowly. The dial moves very slowly. So slowly that I have to sort of stop myself from saying, okay, that's the temperature. I have to wait a little longer. Find something else to do. Find something else to do and then come back. The soil thermometer is like a shank with a meat thermometer on the end, not the digital ones, but a circular dial that faces up towards the sky. And you don't want that to be in direct sun because that itself will heat up from the sun. So you want to make sure that all the temperatures that you're reading is coming from the shank. And the probes are usually 8 to 12 inches long. I have a really long one. I have a short one and a long one. 

Farmer Fred

Well, a real long one would be a compost thermometer. 

Debbie Flower

It is. You're right.  So yeah, you don't need to buy one with a real long shank. It's also harder to store one with a real long shank. Six inch shank is probably good enough. Studies have shown that all the feeder roots of most plants, trees included, are in the top six inches of the soil. And that's where you want the temperature to be correct for the plant to grow. 

Farmer Fred

I didn't realize that with a lot of these dial -type soil thermometers that the nut that is below that glass face is adjustable. 

Debbie Flower

So you can calibrate it? 

Farmer Fred

Yeah, for calibrating. It's called a calibration nut. And according to some instructions I found from a soil thermometer made and sold in Australia, if it has a calibration nut, you fill a glass with ice, add water, and stir. Wait a few minutes, then the temperature in that glass should be 32 degrees. You can then calibrate the arrow on the thermometer to make sure it reads 32 degrees. 

Debbie Flower

Wow. That is an excellent thing to do, to keep your thermometer or any sort of a meter calibrated. I'm not aware that mine has that. Have you ever done it? 

Farmer Fred

I've never done it because I never paid attention to the nut. Because there were never any instructions on the package it came in that it is an adjustable nut. And the dial has a cap, a see -through plastic top on it, so somehow you'd have to get that off. Hopefully it will come with directions.  But if you're buying one, I would buy one that you can calibrate. That makes a lot of sense, and again you would set it to that water and ice, or ice and water in 32 degrees.  You hopefully do not have soil that cold. 

Debbie Flower

Right, hopefully it never - and hopefully the meter, check that before you buy it, goes down to 32 degrees.  They don't all go that low. 

Farmer Fred

Should the soil be moist before you take the soil temperature? 

Debbie Flower

Good question. I want to say yes. I don't know for sure, but you're going to be germinating or planting. You want water in the media for whatever you're planting, but you don't want to go out, turn on the water. Water the garden and then take the temperature because water has its own temperature. And so at that moment, it will be whatever the temperature of the water is, which has to do with how long has your hose has been sitting in the sun? What is your source of water? And how cold is that? You want to know the temperature what over time the plant will be sitting in or the seed will be sitting in. I would water the garden the day before, at least. 

Farmer Fred

Okay, that would make sense. Okay, scenic bypass question. If that hose has been sitting in the sun all day and you are about to water seedlings in the late afternoon and you turn the faucet on and you feel that initial water coming out of your hose, it's hot. It's really hot. It's really, really hot. Is it safe to apply that hot water around plants? 

Debbie Flower

Probably, it depends, which is a common answer around here. What I do is just turn it over an area that has just mulch in it and let it water that area, because you can feel it coming through the hose or if you've got a wand, you can feel it cooling off to go through there. Occasionally, it can get so hot, that one, it can burn the plant, literally. It's not from, plants do not burn from a drop of water on the leaf and the light going through and it acting like a magnifying glass. That just doesn't work, that doesn't happen. If you're getting burn spots on your leaves after watering, it's because the water's too cold or it's too hot or it is too high in some kind of chemical.

So, I just allow that water to go out first and then start or I'll fill a watering can with it. Something like that. I hate to waste water. 

Farmer Fred

Unless it's a black plastic can and it's sitting in the sun. 

Debbie Flower

Yes. Yes. I don't have any black plastic cans, fortunately.  But even the dark green ones, the water doesn't seem to get that hot. 

Farmer Fred

That's a good point. 

Debbie Flower

Water is a conundrum for me because I'm trying to get seeds to germinate and they like warm soil, but they need to stay wet. And so if you water them with cool water, you're reducing the temperature around that seed. And if it hasn't germinated yet, that can cause it to rot. 

Farmer Fred

So you turn on the hose, you put your hand where the water is, and you go hot, hot, hot, hot. And so you wait a couple of seconds and you feel it again. And it's hot, hot, hot. And then a few seconds later, you go, that's pleasant. Then you can water your seeds. 

Debbie Flower

Yes. OK. Or put it in a watering can, leave the watering can outside and then the water in the watering can will be whatever the outside temperature is. 

Farmer Fred

If it's in the shade. 

Debbie Flower

You think it would be too hot in the sun? 

Farmer Fred

I think it depends on the pot. 

Debbie Flower

Okay,  it depends on the watering can. Yeah. Okay. So, if it's in the shade,  chances are it would be the air temperature. 

Farmer Fred

Right. At that point and then you can use that. All right. So, that's a good compromise. Yeah. Use a watering can that you have in the shade. And now back to the main freeway here. So,

make sure the soil is evenly moist perhaps a day or two after watering. Yes. Then it's fine. Then in these Australian instructions I was reading for this particular soil thermometer. It said It's a good idea to plunge a screwdriver into the soil to make a hole before inserting the soil thermometer probe into the soil. That way you won't damage the probe should it accidentally hit a rock. And it also made another point in those instructions too about Don't push it down by banging on the top of the soil thermometer probe or the the top, the glass part. You grab the probe itself and gently push it into the ground. 

Debbie Flower

I've been bad. I've been bad to my soil thermometers and  I haven't pre -drilled the hole. 

Farmer Fred

Well, that kind of makes sense if you have rocks. 

Debbie Flower

Yes. Well, there’s that. There really aren't rocks in the raised bed. 

Farmer Fred

OK. All right. In my case, there are Irrigation pipes. Drip irrigation pipes. I don't know if I'd want to hit those with a screwdriver either. Right. But usually I just dig down with my finger. 

Debbie Flower

Okay, that works. And that would work too. And then you cover it over again? 

Farmer Fred

Yeah. 

Debbie Flower

Because it's got to touch the soil.

Farmer Fred

  All right. So you check the soil temperature at about two inches for seeds, maybe four inches for transplants? 

Debbie Flower

Right, wherever the roots are going to be. 

Farmer Fred

“Are going to be” or “are”?

Debbie Flower

Are. Are. Yes, you're right. Where the roots are. 

Farmer Fred

All right. Because that depends on the maturity of the plant then.  But roots grow amazingly fast. They do. And they probably grow twice as fast underground as what you see on top. 

Debbie Flower

OK. Probably depends on the plant. 

Farmer Fred

But it seems to me that with plant roots, there’s more of a mass of them than there is of what's on top. 

Debbie Flower

OK. Yeah. Maybe. I think it varies by plant. And the roots don't necessarily grow down.

They grow where there is water and oxygen. Right. And so that depends on your media. 

Farmer Fred

Yeah, roots do not head straight down. I think a lot of arborists have taught us that over the years, the fact that tree roots tend to follow the profile of the moisture in the soil. Yes. That's why they extend out for hundreds of feet, possibly, in the case of really mature old ornamental trees. And that's why lawns are never a good idea to have trees in because the lawn is going to get the water before the tree gets it. 

Debbie Flower

Right. Or the roots are going to come up to the surface and be a problem. 

Farmer Fred

Yeah. All right. One thing I would say, Craig, it would be to choose a soil thermometer where there are instructions included.  Detailed instructions include maybe even a book on how to do it. But I'll have a link in today's show notes. Information about soil thermometers from the University of Wisconsin in Kenosha that has a very good soil temperature guide in there that can help you out and answer a lot of your soil temperature questions. All right, we've taken the temperature of that question. Thank you, Debbie. 

Debbie Flower

You're welcome, Fred. 

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Q&A  ALL ABOUT SOIL pH TESTING

Farmer Fred

We answer your questions here on Garden Basics. We got a question from Paige. She is in zone nine in Houston, Texas, which is a little bit of a, a little different zone type nine to be  gardening in, compared to us. We can get into that possibly. We are at an Open Garden Day here at the Fair Oaks Horticulture Center talking with vegetable expert and master gardener, Gail Pothour. And Gail, Paige says she's a brand new baby gardener. I guess she's not growing babies. Well, maybe she is, I don't know.

She says, “I have a multi -use soil probe and have been checking my pH. Why is my pH drastically different from four to six inches down as compared to eight to 10 inches down? When setting up my raised beds, I layered in order cardboard, small sticks and branches, leaves partially broken down from the yard, a soil compost mix from my local nursery. The soil, by the way, had a good soil test besides nitrogen to which I have added blood meal.

And she says, I fertilized about a month later when my plants were established with a 6 -2 -4 slow release granular fertilizer. Now the pH in my tomato bed is 6 .7 at the 3 to 6 inch depth and it's 4 .7 at 8 to 10 inches depths. They seem to be doing okay, some better than others. Is this a problem? It's not a problem if you want to grow blueberries, Paige, but that's pretty acidic soil, Gail.

Gail Pothour

Yeah, the 4 .7 reading, that is very acidic. But a lot of it could be because she has layers, different kinds of layers. It sounds like she's doing lasagna gardening or kind of a modified hugelkultur, where you do all this layering with sticks and things like that. So until that all breaks down, I think you're going to get different readings. Plus, if you're using a multi -use tool, in my experience, they're not that reliable.

Farmer Fred

In my experience, they're definitely not that reliable. I mean, you can get a really good pH testing tool, but they're very expensive and you have to maintain them and you have to calibrate them regularly. And you know, so I just prefer to get a soil test from from a lab, you know, have them tell me what it is now, not to sell them short, but at a nursery, you can buy a separate pH test and they're fairly inexpensive, they're under $10. You can buy an inexpensive soil nutrient test that usually tests nitrogen, phosphorus, and potassium. They're usually one-use products and they may be accurate, but in a lot of situations it's better than the meters you would be buying. 

Gail Pothour

Right, I haven't found in my experience that the meters are that reliable and so if I do a soil test at home I will use the Do It Yourself test kits, which is like a science experiment. And so I do that every year or so to find out what my N-P-K is and what the pH is. And then periodically I try to get a professional test and especially if it's a brand new garden, I encourage you to get a professional soil test. To know, kind of a starting point, so you know what you have. But you need to wait until you're soil has all decomposed and mixed together. You don't have individual layers. 

Farmer Fred

Right. If you're digging down 8 inches, Paige, and you're finding things that you can identify, you don't want to be judging your soil based on that. You want to make sure it's all dark and rich and soil -like. 

Gail Pothour

And it all looks the same, yeah. And you don't want to be able to identify sticks and things like that and pieces of cardboard. Hopefully, you'll get a lot of earthworms in there that will be breaking everything down. But you can add some kitchen scraps, you can add other things, add some shredded up leaves and kind of continually mix that in. But until it gets thoroughly decomposed and mixed together, I don't think you're going to have a reliable pH test. 

Farmer Fred

There's only a handful of universities in the United States that do fairly inexpensive soil tests that you could rank as being complete tests. And Paige, luckily for you, one of them is in your state. It's Texas A&M University, and we'll have a link to their soil testing facility in the show notes. They will have more information on how to take the sample,  before you submit it for soil testing. 

Gail Pothour

Right. There are very definite instructions on taking a soil test and so you do need to contact the lab or whoever's going to be doing it and find out how they want it. Typically, you want to scrape off any organic matter off the top and then you dig down a certain, depth, six inches or whatever they tell you, from different parts of your garden and mix it together, and you take it in. But find out the specifics for where you take it because that can skew the soil results if you haven't taken it correctly. 

Farmer Fred

And you don't want to use any metal objects for digging either. 

Gail Pothour

I have heard that. I haven't been as cognizant of that as I should be. I just use my stainless steel trowel. But yeah, you'd want to be sure that you don't take a soil test right after you fertilize because that could skew it, as well. So find out what the parameters are from Texas A&M if you go that route. Find out how they want the soil prepared before they test it. 

Farmer Fred

Yeah, it's reasonably priced. Texas A offers this test for pH, macro and micronutrients for about $19 and that's a heck of a price when you consider that if you're trying to deal with a private soil test company it could be $100 or more. 

Gail Pothour

Right and yeah that is a great price. They may not test for nitrogen because that's so mobile. I mean  one day you've got a lot of nitrogen; the next day you don’t. So they may not test for that. So nitrogen is something that you'll probably always have to be adding with a fertilizer or cover crops or whatever. But Yeah, I can't stress enough to get a good professional soil test at least once. 

Farmer Fred

But Paige, enjoy your baby gardening. I think you're off to a good start. You're constructing it correctly, and you just have to be more patient and let that soil form, and it'll get there. It'll take time. 

Gail Pothour

And then you'll just kind of, each year need to be replenished with more organic matter. And be sure to mulch when you do plant. I can't stress that enough, especially in a hot climate, that mulch will insulate the roots, they'll keep the soil from getting so hot. You don't have to water as often, it keeps the moisture in. So yeah, mulch, mulch, mulch. 

Farmer Fred

I mentioned earlier that Paige is in Houston, which is in USDA zone 9. We are in USDA zone 9 here in California, as well. The big difference between where we live and where she lives is the humidity. 

Gail Pothour

Absolutely. We are dry and Texas is quite humid, I understand. I've not been there myself, but I have family in the south and it’s a totally different environment when you grow a garden. 

Farmer Fred

Yep, and more diseases as a result. 

Gail Pothour

yeah, in fact I found out my family in South Carolina, they can't grow tomatoes because it's too many fungal infections, so it broke my heart. 

Farmer Fred

It's good news for us, that means Gail's staying here in California. Gail, thanks so much.

Gail Pothour

You're welcome Fred.

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Farmer Fred

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Q&A  ALL ABOUT CARDBOARD SHEET MULCHING

Farmer Fred

We get your garden questions here on the Garden Basics Podcast. We like to tackle them even when they're from Don in Southeast Indiana. He asks a lot of questions, but that's okay. Don says this time around, he says, “my wife and I moved here last summer. There are plenty of projects in this newly acquired house that is our home, including yard work. I want to expand the landscape areas around the property, which typically means digging up the existing lawn or at least stripping the grass or the sod off the top. However, I have heard of covering an area with cardboard to kill off the grass and weeds. I have also heard about using worm castings, which I have none of at this time. I believe people use shredded paper for worm beds while vermicomposting. So why not do both at the same time? I'm contemplating covering this new area, grass and all, with shredded paper, shredded by my office paper shredder, then covering that with the cardboard, then watering it well, then covering it with regular wood mulch, and then planting through all those layers later this fall. What are your thoughts about that?” 

My thoughts are, Don, that's a full -time hobby. You're working too hard. Debbie? 

Debbie Flower

Maybe he likes that. 

Farmer Fred

Yeah, maybe he does. Maybe he wants to be out of the house. 

Debbie Flower

Yeah, but he’s kind of on the right track in that he can smother the grass and create an environment that's going to be good for planting in, but it will take time, as well. First, I would mow the grass as short as possible. And you can leave, assuming there are no weed seeds in that, what you've mowed off, you can just leave the grass clippings on the soil and put in something that will provide nutrition and life to the soil.

So that would be some sort of compost. It could be bagged stuff. There are bagged worm castings, or you can go to a place that sells them in bulk. There are places that have large worm composting happening, and you can buy from them. And then you can put down a layer of cardboard. They're all different kinds of cardboard. There's cardboard that is highly waxed and coated and has lots of colorful ink on it. I wouldn't use that.

Farmer Fred

And tape. 

Debbie Flower

Yes, there's cardboard with tape and metal staples. I would remove all of that. You want cardboard as simple as possible, as unadulterated as possible. The stuff that's shiny will resist water. It'll roll right off. You want something that will allow the water to absorb into the cardboard. You want a type of cardboard that will allow water to absorb into it. And in fact, when I did this with my son at his one of his homes, we watered the cardboard before we even put it down onto the landscape. 

Farmer Fred

Yeah, we'll have a link in today's show notes that show you a step -by -step process for doing cardboard mulching, which some experts even advise against doing as far as using cardboard. Don't use cardboard, just use straight mulch. That's up to you. But these instructions will have what you do step -by -step when applying cardboard. And one of those first steps is watering the cardboard. 

Debbie Flower

Right. You don't want it to prevent water from getting to your soil below and allowing that soil to start to become rich and better for gardening. You want it to just prevent things from growing. But then you would put probably an arborist mulch on top of the surface. 

Farmer Fred

Yeah. Or Don has some big truck come out with worm castings, I guess. Bring that out. 

Debbie Flower

Or mushroom compost can be pretty good. Sometimes it's a little high pH, but that washes out. But then you have to leave it for I would say most of a year. If you're doing it now by planting season next spring, you should be able to plant right into it. But it takes time for the cardboard to break down. 

Farmer Fred

Yeah, it should be really thin cardboard too and one layer. 

Debbie Flower

How do you get really thin cardboard? 

Farmer Fred

Buy a lot from Amazon. 

Debbie Flower

Okay. Corrugated is fine. Corrugated, though, allows that box to be more water resistant. It's dangerous when it's a box that had your new gardening tool in it and it's just a thin piece of cardboard and it's got glossy pictures on the outside. That probably is not what you want to use. 

Farmer Fred

No, plain as possible. 

Debbie Flower

Plain as possible. Yeah. Give that lasagna preparation at least six months to kill what's underneath it and for the stuff to start to compost, including the cardboard. The cardboard should start to decompose in that area and then you can plant right through it. And when you do, you want to not see anything that looks like cardboard. Then it's good to plant through. 

Farmer Fred

And do you keep watering throughout the six months you're waiting, if it doesn't rain? 

Debbie Flower

That's helpful. Yes. Because you're creating life. 

Farmer Fred

Thank you, Dr. Frankenstein.

Debbie Flower

The life that's in the soil will start to flourish because you've given it something to eat, the grass clippings and the compost you've put underneath and eventually as the cardboard starts to break down into its parts, it will eat the cardboard as well. And those live organisms need water. They don't need to drown, but that irrigation of one inch a week rule of thumb in the warm weather can be followed. It doesn't need to be that much as it gets cooler.

Farmer Fred

Don, have fun with that new yard. There's a lot of projects there, so get to it. All right, Debbie, thank you so much. 

Debbie Flower

You're welcome, Fred.

WANT TO LEAVE US A GARDEN QUESTION?

Farmer Fred

Want to leave us a question? You’ll find a link at garden basics.net. Also, when you click on any episode at garden basics.net, you’ll find a link to Speakpipe in the show notes, where you can leave us an audio question without a making a phone call. Or, go to speak pipe directly: speak pipe dot com slash garden basics. You want to call  or text us? We have that number posted at garden basics dot net. it’s 916-292-8964, 916-292-8964. Email? Sure! Send it, along with your pictures to fred@farmerfred.com. Or again, go to garden basics dot net and get that link. And if you send us a question, be sure to tell us where you’re gardening, because all gardening is local. Find it all at garden basics dot net.

Farmer Fred

Garden Basics with Farmer Fred comes out every Friday. It's brought to you by SmartPots and Dave Wilson Nursery. Garden Basics, it's available wherever podcasts are handed out. For more information about the podcast as well as an accurate transcript, visit our website, gardenbasics .net. And there, you can find out about our newsletter, Beyond the Garden Basics. And thank you so much for listening and your support.