How to water plants through a heat wave

How to water plants through a heat wave

Heat waves make me a worried gardener. What can I do to help my plants? Should I water again? How much?

This summer of 2017 has started off with a blast of multiple heat waves for Southern California’s valleys (the beach has been spared by the presence of a marine layer). In my yard, twenty miles from the ocean in San Diego County, it was over 95 for six days June 16 through 21, and then again for three days June 25 through 27. A couple days reached over 100.

But my vegetables and fruit trees came through pretty well, save some sunburned apricots:

sunburned apricots

I believe it’s because I’ve settled on a two-part watering routine that helps the plants cope with heat waves. Here’s what I do:



First, I make a pre-emptive attack. I’m convinced that this is the most important thing to do. When a heat wave is predicted, I pre-irrigate, heavily. I learned this from Reuben Hofshi, an avocado farmer. The day before a heat wave is forecast to start, I water with enough to give all plants full moisture in the soil of their root zone, and then I add about 50% or more. So, for example, I gave my avocado and citrus trees a normal round of irrigation of three hours and 18 minutes, and then I applied an extra 50% of one hour and 39 minutes.

The idea is to make sure the trees have access to all the moisture their roots can touch. They’re totally prepared to pull up maximum amounts of water to supply their transpiring leaves in the stressful heat.


Daily replenishment

Second, I replenish the water used by some plants each day during the heat wave. I feel the need to do this for avocados and vegetables. I’ve noticed that they stress if I don’t. On the other hand, citrus and deciduous fruit trees (apricot, peach, pomegranate, plum, apple, etc.) don’t need such coddling. I don’t water them daily through a heat wave.

But for avocados and vegetables, I estimate how much water they’re using each day and replace it each day. For example, I gave my avocados 42 minutes each afternoon.

Meanwhile, for my vegetables, before the heat waves I had been watering my vegetables for 25 minutes every three days, which had been working well, and which is equal to about eight minutes per day. During the heat wave, I ran the drip lines on my vegetables for 12 minutes each day; that was my estimate of how much they needed. I came up with that time by taking the daily eight minutes and adding 50% (four minutes) since it was hotter than usual, which gave me 12 minutes.

To sum up, I pre-irrigate with at least 150% of what I think the plants need the day before the heat hits in order to be fully saturated, and then I irrigate every day during the heat with about 150% of what avocados and vegetables normally need each day in the summer. 


Nota bene:

Let me mention some factors about my specific situation and why they might make things work differently for you. My soil is sandy loam. If yours is clay, you shouldn’t need to water anything daily even in a heat wave. Also, I irrigate my vegetables with drip lines. When I used to irrigate with overhead sprinklers, I could get through heat waves without watering my vegetables daily. The sprinklers gave them larger root zones and therefore made for tougher plants.

I have always watered my deciduous fruit trees every ten days to two weeks in the summer. It is when a heat wave occurs that I’ll water every ten days; otherwise, during normal summer temperatures, it’s every two weeks. Deciduous fruit trees (and grapes) never stop surprising me by how little and infrequently they need water — while still producing oodles of great fruit. That’s why I keep planting more of them!

grapes on chainlink fence


P.S. Apologies for the pretentious “nota bene,” but whatever, I’ll leave it in there.


You might also like to read:

Gardening benefits of a hot-summer climate

Heat tolerance of avocado varieties

Get your hands dirty: discover the truth about your irrigation practices

Get your hands dirty: discover the truth about your irrigation practices

How do you know if a plant needs water? You can look at the plant’s leaves and make a guess, and you can get a rough idea based on how you’ve watered it in the past.

And if you’ve just watered a plant, how do you know if you’ve given it enough? You can estimate the number of gallons it needs using the size of the plant and the season, and taking into consideration your soil type.

But there is one sure way to answer both of these questions, and that is to put a knee down and start scratching. The truth about whether a plant needs water or whether you’ve given a plant enough water is only to be found by getting your hands dirty, by digging into the soil where the roots of the plant are.

I’ve made a personal commitment to getting my hands dirty more frequently this summer. The other day, June 10, I took some photos as I went through the soil-moisture discovery process for a couple of fruit trees that I was thinking might need water and for some vegetables that I had just given water. Follow along.


Should I water the fruit trees?

apricot tree with mulch

Blenheim apricot

I suspected that my deciduous fruit trees might need water. It had been 20 days since my last irrigation, on May 21. I scraped away the mulch and dirt under this apricot tree until I encountered roots. The shallowest roots were about one knuckle (about one inch) below the soil surface, here:

apricot tree shallowest roots

I scooped up a handful of the soil there, squeezed it in my palm, and then opened up to look at the dirt. People usually call this the soil moisture “feel method.”

soil moisture feel test apricot tree

Immediately, when I opened my hand the dirt broke apart. That meant there was very little moisture in it, which meant yes, I should irrigate my apricot tree today.

Next, I walked over to my Pinkerton avocado tree. Should I water it today? I guessed not, since I’d watered all of my avocados only five days prior, but I had to be sure.

soil moisture feel test Pinkerton avocado

Same process: scrape mulch and soil until encountering shallowest roots. However, this time the soil kept its form in my open hand. I bounced it and it still didn’t break apart. Stains were left on my palm from the moisture in the dirt. This meant that the soil still had plenty of moisture in it. No need to irrigate yet.

My soil is a sandy loam. If yours is a clay soil, it would react a little differently in your hand when you squeeze and bounce it. Here is a document from the U.S. Department of Agriculture to help you gauge the moisture in your specific soil. And here is a video from U.C. Davis to help you figure out your soil type, if you don’t know.

Also, here is a video from U.C. Davis showing the process of feeling for moisture in the soil, just as I did in the photos above.


Did I water enough?

Then I wanted to know if my regimen of drip irrigation on my vegetables was sufficient. Just that morning I had run the drip lines in my vegetable garden for 25 minutes and wondered specifically if that had wet deeply enough to satiate all of the roots of a bed of tomato plants. So a few hours after watering, I got my hands dirty again.

I used a shovel to dig a hole a foot deep between two tomato plants. Then I grabbed a handful of dirt about six inches down and squeezed and bounced it. Stained, held form: wet soil.

soil moisture feel test tomatoes six inches

Next, I grabbed a handful near the bottom of the hole and squeezed. The soil ball broke in two when I opened my hand and bounced, but it wouldn’t crumble further. So the soil had moisture down there but wasn’t thoroughly wetted by my 25-minute irrigation.

soil moisture feel test tomatoes twelve inches

That was fine because I also noticed during my digging that there were no roots that deep. Maybe there were roots a foot down directly below the plant, but over here they went no deeper than about six inches.

My take-away was that I had certainly watered the tomatoes enough, and next time I might water a little less so as not to waste any through deep percolation. I could water less by reducing the time to 20 minutes, or I could lengthen the interval between irrigations.

There’s no substitute for the proof you hold in your hand when you use the “feel method” to discover the moisture in soil. You’re holding the truth when you get your hands dirty and see and feel the dirt right there around a plant’s roots. It makes me feel confident every time I do it.


You might also like to read:

How long to run drip irrigation on vegetables

My soil, in depth

What is the best time of day to water your plants?

What is the best time of day to water your plants?

Looks like the irrigation season is upon us. My yard hasn’t been rained on in over a month now. As you start to water your plants for maximum yield and efficiency you might wonder, as I often have, what time of day is best: morning, afternoon, evening, middle of the night?

Whenever you can be watching — that’s my operating principle.

When I say this I’m thinking of what time of day it’s best to run an irrigation system, like a set of drip lines in a vegetable garden or mini-sprinklers under fruit trees. And what I mean is that it’s best to run that system at a time when you are usually home and able to observe. For example, if you leave for work every day at 6 a.m., then it’s best to run your irrigation at maybe 5 p.m. when you’re home from work, not at 7 a.m. while you’re gone even though both 7 a.m. and 5 p.m. are perfectly fine times of day to water plants.

I think that being able to watch your irrigation system run is vital because of all of the things I’ve noticed over the years while watching my systems operate. I’ve noticed clogged drip emitters, mini-sprinklers that needed a flick to get them spinning again, a leak in a valve, a drip emitter that I forgot to plug up after the plant it was watering died, and on and on.

My system was running just the other day when I heard a curious spraying sound. I dug to find this:

Irrigation leak

If you’re around to watch while your system is running, then you can catch these things red-handed rather than read the evidence of a dried-up plant or a mossy spot in the soil after the problem has been long occurring out of sight.

If you’re watering by hand using a hose or sprinkler or watering can, then of course you’re going to be watching, so is there a best time of day for that? Morning and evening are both great there too. I’d say that only noon in the middle of summer is not ideal, and that’s because you’re going to lose a little more water to evaporation at that time. But who would choose to stand in the sun in the middle of the day in summer anyway?

One thing we don’t need to worry about here in Southern California is the idea that watering in the evening or at night and wetting the leaves of plants might encourage fungal diseases. That might be a consideration in more humid climates, but it’s a non-issue here. I’ve watered and wet leaves in the evening some years and never wet leaves other years, and I’ve seen no difference in the incidence of diseases like powdery mildew and downy mildew — and that’s both in my gardens near the ocean and in the relatively drier inland areas. My observations have been that mildew hits plants like peas and squash as they age regardless of the watering regimen, and the occurrence of botrytis on blackberries is mostly due to insufficient sunlight, not wet foliage and berries.

mini-sprinkler under avocado tree

What do I currently do in my yard? I run mini-sprinklers under my fruit trees in the late afternoon, usually starting around 4 p.m. I like this time because the common west-northwest breeze coming from the ocean is starting to slow down, but it’s still not totally calm; having some movement in the air helps distribute the sprinklers’ droplets a little more broadly.

And I run drip lines in my vegetable garden in the morning around 7 a.m. This is because I’m often at home at this time, and I can “walk the lines” briefly to check on them.

When can you be watching? That time is the best time for you to water your plants, in terms of their health and the efficiency of your water use in the long run.


Free talk on rainwater collection

Free talk on rainwater collection

If you live in the San Diego area, you might be interested in attending a talk I’ll be giving in two weeks on rainwater collection. Should’ve done this two weeks ago, I know. Feels like we’re getting our entire winter’s worth of rain right now. Above is what my rain gauge collected in the last three days, and there’s more on the way.

The talk is free; I’ll be doing it as a UCCE Master Gardener volunteer. It’s on Saturday, February 4, 2017 at 1:30 pm at the Del Mar branch of the San Diego County public library.

I’ll be sharing reasons for rainwater collection as well as my experiences with different methods for collection and use in the garden. I’ll show lots of photos. In addition, I’ll talk a bit about using graywater (reusing household water).

Rain makes dreams come true

Rain makes dreams come true

This morning I awoke to the sound of rain. It’s the most pleasant sound, a muffled pecking on the roof. This is the first storm in a series of four that the National Weather Service says will give us three to six inches by Tuesday, an event with more potential to flood than anything we’ve seen since 2010, before the drought started. (Will this be the first year since then that we have above average rainfall too?)

I am possessive of my rain. See, I just did it — “my” rain. If I’m home while it’s coming down, then I’m outside running around checking levels on the tanks, adjusting hoses, observing the mulched basins I’ve built, getting soaked. My goal is that every raindrop sinks into the soil on my property — that not a single drop runs off my property.

I’m not there yet. I still lose some from the driveway, whose curb I need to make more cuts in. But elsewhere I capture it all. I refuse to treat rain like sewage.

How can we lament the drought and demand watering restrictions only to discard rain into the streets?

Recently, I cut a downspout pipe that channeled rain from one section of my roof directly to the street. It used to be that the rainwater gushed out of that pipe and eroded the soil at the edge of my yard and then carved a gully out of our dirt road and then flowed downhill to add to our neighbors’ flooding problems. So I unearthed that pipe and chopped it.

For one, that rain is mine. I’m going to keep it. For two, that erosion and flooding is my fault and I need to knock it off.

When my wife and I were looking for a house to buy we dreamed of having a yard large enough that would allow me to grow much of our food, and we also dreamed of having a creek running through the property. But creeks are extremely rare in Southern California, and alas we moved into this house and left our dreams of a creek behind.

Below our house the yard slopes down at about 5 degrees. One day after chopping the downspout pipe, I put our two sons in a wagon and told them we needed to collect rocks to make a river bed. Cass liked the idea of “making a river.” The boys and I tossed rocks into the wagon and then I towed them back to the place where I’d chopped off the downspout pipe. At the opening of the pipe they eagerly helped me lay a path of rocks — some yard work is meant for young boys, and carrying and tossing rocks is one of them.

“We made a river!” I said.

Well, we had made a bed for a tiny creek. Cass didn’t call me out on it.

Our creek is very short and it is not perennial; it only runs during a rain. But an ephemeral creek is better than none, and it’s better than erosion, and it’s better than flooding our neighbors, and it helps sink “our” rain into our yard. Better yet, it sinks the rainwater between some of our fruit trees, and I imagine the trees’ roots will eventually tap our creek’s flow.

Our creek is flowing this morning. And it looks like it will continue for the next week straight. Rain is making our little dream come true.

How long to run drip irrigation on vegetables

How long to run drip irrigation on vegetables

The thorniest question.

My short answer is: Start with something like 45 minutes, observe, and adjust. But there are a lot of variables, which is why it’s a thorny question, and which is why observing and adjusting is necessary. Read on for details and reasoning and experience.

I gave a presentation on drip irrigation at the Gardening with Class conference for school gardens on Saturday, put on by the Master Gardeners of San Diego County, and I had straightforward answers to most people’s questions such as, “When should I run the system?” (Whenever you can watch it. In other words, just not in the middle of the night. This is because it’s harder to know if an emitter is blocked or if a poly tube has been punctured if you aren’t able to watch the system run.)

But how long to run drip irrigation?

“Uh . . . it depends,” I said.

There are many factors to consider. Emitters range in volume from 0.5 gallons per hour to 4 gallons per hour, or even wider. So you can imagine that running a system for one hour will give plants a very different amount of water according to the type of emitters used.

Then there is the number of emitters. There are drip lines with inline emitters spaced every 6 inches up to every 18 inches, or even wider. So if a broccoli plant is drinking from multiple emitters under it on a drip line that has emitters spaced every 6 inches, you will need to run that system for a shorter duration than if the emitters were spaced every 18 inches.

On top of that — or under it, actually — is the soil type. In a raised box for vegetables that is filled with potting mix-type soil, a drip system should not be run as long as for vegetables planted in the ground in a clayey soil. A clayey soil can soak up a lot of water and hold onto it, but a loose and light potting mix-type soil lets water percolate down, down, down to below where a plant’s roots will reach.

And there are even more variables, but how could I answer to help people at least get started? I referred them to a handy reference table in a booklet published by the irrigation supplies manufacturer, DIG. It says that this time of year, October, a drip system on vegetables should be run for 30 minutes to one hour every three days.

I also mentioned that I’m running my own drip irrigation on my vegetables for 30 minutes every three days on average in October, and I have 0.5 gallon-per-hour emitters spaced every 12 inches, and my soil is a sandy loam (plants are in the ground). There’s your starting point.

I should have noted that in the height of summer, July, I was running the same drip system for 30 minutes every two days (every other day). My summer vegetables did well on this regimen.

I finished my answer by saying that a good way to get started is by shooting in the middle, say 45 minutes every three days, and then watching how the plants respond. Is there any wilting, especially after the sun cools in late afternoon? Then raise the time to an hour or decrease the interval to two days and see if it makes enough of a difference. Or does the surface of the soil still appear wet after the three-day interval (when it’s time to run it again)? Then decrease the time to 30 minutes or lengthen the interval to four days.

If the surface of the soil still appears wet (dark in color) after your scheduled interval, then you shouldn’t water again yet because you’ll be watering too frequently and either wasting water or drowning your plants, or both. Dig down and you’ll find that most vegetables in most soils don’t have roots within an inch of the soil surface. Moisture in that upper inch is therefore nearly useless to the plants. That upper inch of soil is also quite vulnerable to evaporation from the sun. Wait to water again until the soil is no longer wet where the vegetable roots are — that’s an inch deep and deeper.

The short of it: Start with something like 45 minutes every three days, observe, and adjust.

This is for now, October. In winter, I’d start from 45 minutes every five days and be at the ready to stop the system from running if there’s rain. In spring, I’d start from 45 minutes every three days (just like fall). In summer, I’d start from 45 minutes every two days.

There. The thorns have been pruned off. Naw, just dulled a bit. But it’s a start.


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