Transpiration
I'm still learning about gardening in the arid southwest.
I knew that the little plants I just put in my newly created gardens would need frequent watering at first. I added compost to hold water, and planted things that will take drought, but was prepared to be out there with the hose several times a week in the beginning. But I had no idea.
It turns out they need water, and a lot of it, every single day. Everything. Even the rosemary and the lavenders and the dry loving penstemons. And even with daily watering, in the hot breezy afternoons they wilt. The soil is so powdery dry so far down and so wide afield that a soaking around the small roots of my tiny plants gets wicked away.
But the bigger issue is transpiration. The air is so dry, even at night, that the good, deep, daily soaking I give the roots is quickly released through the leaves. Even in shade, shielded from the hot New Mexico sun that bakes the soil, the air creates a humidity gradient that pulls moisture from the hydrated leaves into the dry air.
It's not evaporation. Water isn't simply drying off the leaves. Transpiration is physically pulling the water from the soil out into the air through the leaves' pores.
Plants need a humidity gradient to move water around. That's what pulls the water up from the roots and into the plant's leaves. In any climate, they need to transpire through their leaves, releasing moisture into drier air in order to get water up out of the soil where the leaves can use it.
Transpiration is slowed by the cuticle layer of the leaf which is the waxy or glossy coating on leaves. Rosemary has its fragrant oily coating, and narrow leaves as well, to slow water loss. Lavenders do too. My western redbud, Cercis texensis, has very shiny leaves from a glossy coating. Aspens have a thin stiff coating to prevent water loss, and like oak leaves they are impossible to compost -- the coating keeps fallen aspen leaves from ever breaking down.
I really notice the transpiration issue with some plants that have thin leaf cuticles. I decided to put epimediums under the pine tree in front. They are a beautiful woodsy looking ground cover that likes dry shade. Perfect for my Santa Fe garden. But the leaves are paper thin, which gives the plant a pretty, translucent look despite the fact that they are tough and need little water.
Epimediums failed when I planted them here. I put them in shade, I kept the soil quite wet, and they immediately transpired themselves to death. The leaves are too thin, the water too quickly moved from the wet soil out into the air, and my dream of recreating a shimmery ground cover in my garden here was quickly dashed.
New Jersey Tea (Ceanothus ameicanus) is another tough, dry loving plant that does well in lean soil and seems perfect for Santa Fe. I grew it in Connecticut and it actually did poorly because the soil was too damp and too rich.
New Jersey Tea is sold at the garden centers here, and I do have hopes that I can get mine established. But it is not a western native, and the leaves are thin. Without that southwest adapted thick cuticle or glossy covering, the tiny leaves of my first tiny plant withered despite lots and lots of water.
My new plants and I seem to be in the water transport business. I water and water and they move all I give them into the atmosphere before the day is over. I do it again the next day and they move it into the air. Transpiration is so rapid that the plants aren't putting on much growth or able to use nutrients from the soil very much. They are struggling.
We are locked in a daily routine of putting gallons of water into the soil and sending it out onto the breezes. And yet, for all that recycled moisture, the air in my gardens remains, stubbornly, at 10% humidity. Sometimes less.
I knew that the little plants I just put in my newly created gardens would need frequent watering at first. I added compost to hold water, and planted things that will take drought, but was prepared to be out there with the hose several times a week in the beginning. But I had no idea.
 |
| One day this hot dry strip under the kitchen window will be three tall wavy grasses flanked by a sweet lavender on the left and a big rosemary on the right. Right now they are tiny, thirsty, struggling little wisps. |
It turns out they need water, and a lot of it, every single day. Everything. Even the rosemary and the lavenders and the dry loving penstemons. And even with daily watering, in the hot breezy afternoons they wilt. The soil is so powdery dry so far down and so wide afield that a soaking around the small roots of my tiny plants gets wicked away.
But the bigger issue is transpiration. The air is so dry, even at night, that the good, deep, daily soaking I give the roots is quickly released through the leaves. Even in shade, shielded from the hot New Mexico sun that bakes the soil, the air creates a humidity gradient that pulls moisture from the hydrated leaves into the dry air.
It's not evaporation. Water isn't simply drying off the leaves. Transpiration is physically pulling the water from the soil out into the air through the leaves' pores.
 |
| A droopy butterfly bush (Buddleia) by the kitchen door. It's well watered and will perk back up, but it's losing more in transpiration in the afternoon than it can replace. |
Plants need a humidity gradient to move water around. That's what pulls the water up from the roots and into the plant's leaves. In any climate, they need to transpire through their leaves, releasing moisture into drier air in order to get water up out of the soil where the leaves can use it.
Transpiration is slowed by the cuticle layer of the leaf which is the waxy or glossy coating on leaves. Rosemary has its fragrant oily coating, and narrow leaves as well, to slow water loss. Lavenders do too. My western redbud, Cercis texensis, has very shiny leaves from a glossy coating. Aspens have a thin stiff coating to prevent water loss, and like oak leaves they are impossible to compost -- the coating keeps fallen aspen leaves from ever breaking down.
 |
| My western Redbud, drooping in the afternoon heat despite thorough soaking and a glossy cuticle coating on its leaves to slow water loss. |
I really notice the transpiration issue with some plants that have thin leaf cuticles. I decided to put epimediums under the pine tree in front. They are a beautiful woodsy looking ground cover that likes dry shade. Perfect for my Santa Fe garden. But the leaves are paper thin, which gives the plant a pretty, translucent look despite the fact that they are tough and need little water.
 |
| Epimediums in fall in my old garden |
Epimediums failed when I planted them here. I put them in shade, I kept the soil quite wet, and they immediately transpired themselves to death. The leaves are too thin, the water too quickly moved from the wet soil out into the air, and my dream of recreating a shimmery ground cover in my garden here was quickly dashed.
 |
| Papery Epimedium 'Frohnleiten' leaves are thin, and went from leafy transplant (on the left) to potato chips (on the right) despite deep and frequent watering. |
New Jersey Tea (Ceanothus ameicanus) is another tough, dry loving plant that does well in lean soil and seems perfect for Santa Fe. I grew it in Connecticut and it actually did poorly because the soil was too damp and too rich.
 |
| New Jersey Tea in my old garden did well for a season or two. But the soil was too damp and too rich and it declined after a few years. |
New Jersey Tea is sold at the garden centers here, and I do have hopes that I can get mine established. But it is not a western native, and the leaves are thin. Without that southwest adapted thick cuticle or glossy covering, the tiny leaves of my first tiny plant withered despite lots and lots of water.
 |
| My second attempt at New Jersey Tea. The first was well watered but its thin leaves crisped to death. This one gets even more water every day. It's doing better, but has slightly chlorotic looking leaves. |
My new plants and I seem to be in the water transport business. I water and water and they move all I give them into the atmosphere before the day is over. I do it again the next day and they move it into the air. Transpiration is so rapid that the plants aren't putting on much growth or able to use nutrients from the soil very much. They are struggling.
We are locked in a daily routine of putting gallons of water into the soil and sending it out onto the breezes. And yet, for all that recycled moisture, the air in my gardens remains, stubbornly, at 10% humidity. Sometimes less.
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