How Deep Is “Watering Deeply”?

Sunburned and frazzled Celebrity tomato, but still producing nicely!

Sunburned tomatoes left on the plant to continue ripening and shielding remaining tomatoes from direct sun. When harvesting, just cut off the sunburned portion - the rest of the tomato is unaffected and delicious!

Blossom-end-rot on baby tomatoes that I left on the plant in the hopes of their developing further. But, too little of the edible flesh is left, so I'll remove them so the plant can expend its energy in setting new tomatoes instead of "repairing" these.

There's hope! New blossoms are appearing on several plants. We'll see if they survive this coming heatwave and actually get pollinated and develop fruit.

Zephyr bicolor squash on the left, and one Early Summer Crookneck on the right. My husband doesn't like squash - "They're too squashy!" but will eat them sliced raw in a salad. I love my old-variety crookneck, which is more buttery-flavored than the newer straightneck varieties. I enjoy them so much that I can't get beyond the basic preparation of steaming and adding a small pat of butter. Yum!

Cane begonia thrives in the heat, although with just a bit of filtered morning direct sun!

Figs are loving the heat, too, even with some sunburnt leaves. Don't remove the burnt foliage - it'll shade and protect the foliage beneath it.

Bougainvillea loving the heat!

Mulberry blossoms pollinated, even with sunburnt leaves. We'll see if they survive....

Deliciously fragrant frangipani - plumeria - also loving the heat.

Amarcrinum loving the heat!

Dwarf plumeria only 18" tall and blooming for the first time, and with many, many more to come!

Fragrant gardenia also thrives in the warmth, but also in shade with only filtered morning sun.

Letting an artichoke go through its bloom cycle - I love that exquisite "black light" purple! When it's completely dry, I'll harvest its seed.

   The frequent emphasis on “watering deeply” can be meaningless when you don’t know what “deep” means.  Does this mean 1 inch or 1 foot or 3 feet deep?  It all depends on which plants you are growing and what kind of soil your garden space (or container) has.  The point is to get the water to go a couple of inches further down than the roots of your specific plant so that the entire root system is kept consistently hydrated so the plant can grow well. 
 
Which Plants Grow How Deeply?
   Genetics determines how deeply roots grow. Here are some examples:
12 inches = Beets, lettuce, onions, radishes, spinach, and Swiss chard
14 inches = Broccoli, Cabbage, Cauliflower
16 inches = Cherry tomatoes, eggplant, peas, peppers
18 inches = Beans, cucumbers, potatoes, squash, large-fruit tomatoes
   So these depths – plus 2 inches -- are your target watering depths.
 
What kind of soil?
    The kind of soil you have determines how deeply the water will go.

• In sandy soil, with big particles and air pore spaces, water will go straight down and quickly drain away beyond the root zone.
• In silty soil, with smaller particles and air pore spaces, water will billow out more broadly and then drain away.
• In clay soil, with tiny particles and air pore spaces, water will spread out just under the surface of the soil and drain very slowly, taking a much longer time to get down to plant roots.

 
How long should I water?
    The answer to both questions goes back to your soil type. 
    Here’s how to determine what your specific soil texture's absorption rate is:

• Water for 15 minutes
• Wait until the next day to allow the moisture to soak down
• Push a shovel blade straight down into the moist soil
• Gently push the blade forward to reveal how far down the moisture went.  The moist soil will appear darker than dry soil.
• Now you can see how far down the water went with the 15-minute watering.
• Gently push the soil back in place.
• Repeat with longer watering periods – like 30 minutes or 1 hour – until you observe that the water has gone down to the depth you’re looking for for those specific plants.

   Now you know how deeply the water goes with that specific length of time watering.  This length of time will remain constant throughout the year, since it’s based on your soil’s texture and absorption rate.
 
What about shrubs and trees?
    Feeder roots of most shrubs and trees are within the top 12-18 inches, so this is the depth to be sure to keep hydrated.
 
How often should I water?
    Water when soil is only slightly moist at a depth of 4 inches (about the length of your index finger) under the mulch so these top feeder roots remain moist and viable.  The soil further down will remain more moist.
    Each season will change the frequency required to get water to that desired depth. In general, Spring may require watering once every two weeks, Summer once or twice a week, Fall once every two weeks, and Winter once a month. 
   But your soil, your specific plants, and the weather will be the real determiners.  Which is why when the air temperatures are over 100, we water more frequently.
 
Forecast for 100+ for this coming Monday through Thursday
   Since last weekend’s torrid 113 degrees, air temperatures in my garden have been in the high 80s, and I’ve watered deeply every fourth day. 
   With the current forecast for more than 100 degrees for this coming Monday through Thursday, I’ll water everything deeply on Sunday so it’s just ahead of the extreme heat.  I'll water again on Wednesday, and again next Saturday. 
   Each time, I’ll include sprinkling the tops and undersides of foliage as well as the root zones so leaves can absorb additional moisture, and any heat-loving pests like spider mites will be discouraged from settling in.
   I’ll also check the garden each evening to determine whether specific plants need extra water.  My 9" long moisture meter (about $10 at Orchard Supply Hardware), will give me a more accurate reading of soil moisture levels so I don’t end up drowning my plants, assuming they need more water when in fact they don't!

Mulch, Mulch, Mulch!
   And, of course, remember to maintain a 2-inch thick layer of mulch on top of the irrigated soil to lessen evaporation of that treasured water!