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!

​   After several days of more than 110° heat, some of my garden plants were obviously suffering, with shriveled and bleached foliage.   Even though I had watered deeply several days before – as I’d urged you all to do, with my special email -- in anticipation of the forecasted heat, some plants were damaged beyond just a few burned leaves.  The plants just couldn’t adapt to the 30-degree increase in one day.       Immediate and longer-term help was needed.  Here are some guidelines to what I did, and what you can do now and for the rest of the summer.
 
Leave the damaged foliage on the plants. 
   You don’t really know how much was actually killed, so you don’t want to cut off still-living tissue that can perhaps resprout.  Regardless of how it looks.
   The dead foliage serves as a buffer to protect the only-damaged-but-prospectively-surviving tissue from further damage. 
 
Don’t do any pruning at all for the rest of the summer. 
   The plants have already been severely stressed by the extreme heat, and any pruning will add to more stress, requiring the plant to use even more energy to redirect its growth because of what you’ve pruned.
   Allow the plants to use all of their remaining energy to grow however they will. You can modify growth later when the plant is back in full health.
 
Keep plants fully hydrated, both roots and foliage.
Roots -- Water deeply 1 to 2 feet deep. Check how deep the water went with a soil probe or water wand (available at most nurseries and big-box stores).
 
Foliage -- Drench both top and underneath surfaces of foliage, including the “dead” ones.  Leaves that are still functioning, even minimally, will absorb the moisture to augment what’s pulled up by the roots (which may also be damaged from the heat).  Water when the sun isn’t shining directly on the leaves.  Water early in the day so leaves will dry by sunset (so you don’t encourage fungal diseases, which take only 6 hours to develop).
 
Tomatoes and other vegetables
   If you didn’t plant them in a 3-inch depression to use as a watering hole, gently pull back soil to form a berm 6 inches out from the stem.  Fill 3 times with water to make sure the soil is fully moistened at least a foot deep (Tomato roots can reach 3 feet down, if there’s sufficient water).
   Bury a 5-gallon container with holes in the bottom (like what plants and trees are sold in from the nursery), leaving the top 4 inches out of the soil.  Fill 3 times.
   The water from the top and the water coming out of the container holes at the bottom will meet and keep the entire root zone moistened.
 
Fruit trees
   Gently pull back soil to form a 2”-high berm 2-3 feet out from the trunk.  Fill 3 times.  Check that the water has gone down at least 18-24”.
 
Don’t fertilize again for the rest of the summer. 
   Because root systems have been compromised by the heat, forcing them to try to absorb fertilizer will further damage both roots and foliage.  Let the plant recuperate and be actively growing before you demand that it go back into high production by applying fertilizer.
 
If extreme heat is again forecast
   Cover foliage with shade cloth or cheesecloth or other commercial product like Agribon that will lessen the sun’s intensity but let in air and water.
 
Future Production of Tomatoes
Blossom-End Rot
   Expect blossom-end-rot on existing fruits, since that tremendous heat literally sucked moisture from that “endpoint” of the fruits.
   If fruits have blossom-end rot but are almost ripe, let them remain on the plant to perhaps further ripening.  But, after a couple of days, you may find that the rot just gets worse, so harvest and eat the portion you can.
   If fruits with blossom-end rot are tiny and green, remove them since they’ll not be able to grow fully and ripen.
 
No new blooms for a while
   Expect no new blooms, or blooms that don’t set fruit, for a couple of weeks. 
   When air temperatures consistently stay below 85-90° for about 10 days, the plant hormones will again stimulate blossoming.