The damage to plants from extreme heat spikes this June and July is now obvious. Mature drought and heat-tolerant sycamores that did not receive regular deep irrigation have lost two-thirds of their leaf canopy. Crape myrtles that should be in full bloom in summer have lost nearly all of their blossoms. Lawns grasses have turned to yellow straw, rose petals look fried (they are) and fruit trees are dropping most of the immature fruit well before harvest time.

Consecutive weeks of daytime temperatures exceeding 100 degrees and nighttime temperatures remaining in the high 70s have killed many plants and trees and weakened or stunted many others. Determining whether severely heat-stressed plants and trees are dead or dying and whether they have a chance to recover vigor and good health will be a challenge during the next few months. Branch dieback, premature leaf drop, dropping of immature fruits and nuts, dead leaves, blossom drop and flower desiccation as well as yellowed or brown lawn grasses are all signs of severe heat stress as well as drought stress. 

Determining the extent of the heat damage this next month will determine whether a plant or tree is salvageable. Dead leaves don't necessarily indicate that a plant has died. Remove dead leaves on flowering annuals and summer vegetables and scratch the stems to check for green tissue underneath the top layer of bark or stem tissue. Green tissue is still alive. Check leaf nodes for swelling which indicates that new leaves are forming at the node.

Keep the soil moist and delay removing plants which show signs of life. They may well recover and begin producing again in late August.

Protection from the scorching sun this month will speed recovery when temperatures cool in fall. Try to provide full shade this month for damaged plants or plants that do not recover easily from wilting. Move container plants into full shade. Place market umbrellas and shade structures where they can provide the most shade and leave them there. 

If major structural branches on mature landscape trees and fruit and nut trees are dead, the remaining branch scaffolding may not be strong enough to hold a crop or may create a hazardous imbalance. Branches that have lost their leaves or which still hold crispy brown leaves may look dead but still be alive. Remove any dead branches that can become projectiles during fall storms, but wait until deciduous trees are dormant in mid-winter to restructure branch scaffolding. Continue to deep irrigate trees and bushes into the fall months whenever a heat spike is predicted. 

Wait to fertilize all heat-stressed plants and trees until late August when nights will be longer and a little cooler. Deep irrigate before and after fertilization. It will be tempting to feed heat-stressed plants heavily to encourage rapid new growth but feeding at half the recommended rate for the next couple of months will encourage a slow and steady recovery. Apply low-nitrogen fertilizers on landscape plants and bushes and on summer vegetables. Give fruit and nut trees one feeding of a high-nitrogen fertilizer after harvest or in September if there was no crop left to harvest. 

             

Here in the Central Valley, summer heat spikes are becoming more frequent and lasting longer with extremely high daytime and nighttime temperatures. Regular deep, slow irrigation of mature landscape and crop-producing trees is key to keeping them in good health during heat spikes. The goal is to maintain soil moisture in root zones at a depth of 1 to 2 feet. 

We need to deep irrigate more frequently than the previously recommended once a month and deep irrigation must begin at least one day before temps are predicted to be above 95 to 100 degrees for several days.

Check soil moisture levels underneath large landscape trees and deep irrigate whenever the top 5 to 6 inches of soil is dry. That could be several times a month from June to October. Younger trees and bushes planted within the last three years also may need several soakings a month. Soaker hoses are the most efficient and effective means of delivering deep slow irrigation at the outer edges of the canopy where the feeder roots lie but small oscillating sprinklers will also work. Drip emitters are not as effective. It will take at least 3 to 4 hours of slow irrigation for water to penetrate and soak the root zone. 

Insufficient water is the most common cause of poor crop set and premature fruit and nut drop. Fruit and nut trees need regular deep irrigation to maintain consistently moist soil from bloom until harvest time. Citrus trees need sufficient water in spring to set fruit and regular deep irrigation in summer through fall in order to hold and produce good-sized, juicy fruit.

During heat spikes, monitor soil moisture levels under citrus trees and deep irrigate whenever the top three to four inches of soil has dried. Plan on irrigating citrus every 3 to 4 days during heat spikes in July and August and once a week during the warm fall months.

Cool-climate redwood trees have been heavily planted in the hot and arid Central Valley to provide dense shade. Often in spaces too small for the mature tree. As a common practice, redwood trees are thinned and lower branches are removed. The heavy branch structure on redwoods creates a cooler microclimate in the interior of the canopy and thinning and removing branches on redwoods exposes the trunk and bark to the hot rays of the sun and high temperatures.  

In July, the inner needles on redwood trees will turn brown and fall. It's a normal process referred to as redwood dieback. The fallen needles form a thick mulch that will help cool the soil for these shallow-rooted trees and also slow evaporation. Redwoods will benefit from increased deep slow irrigation in summer.

Branch tip dieback is the earliest and most visible sign of drought-stress. Even one long-lasting heat spike without sufficient irrigation can cause tip die back on any type tree. Gusty winds now often accompany heat spikes. Dead branches can become hazardous projectiles and should be trimmed off when noticed, but corrective pruning to reshape branch scaffolding should wait until deciduous trees are dormant in late fall and winter. 

After two consecutive years of normal and above-normal rainfall, trees, hedges and bushes whose growth was constrained during the drought years have shown a huge growth spurt this spring. My neighbor's magnificent loropetalum hedge is at least four feet taller and wider. The small ligustrum trees planted alongside my back fence increased twofold in size so that their branches reached into my neighbor's rain gutters; the flower clusters were enormous. The Meyer lemon tree in the center of my small backyard grew so large that the branches completely blocked the pathways.           

It's so nice to have a lush garden again (however short the time until drought resumes) but the vigorous plant growth will cause problems this summer.

Plants and trees that have suddenly outgrown their spaces this spring will obviously need to be trimmed back, but the trimming and pruning should not be a one-time major cut back. Reducing the size of the leaf canopy by more than 25 to 30 percent during the growing season removes too much of the plants' and trees' sources of energy and can cause stunted growth. A light pruning using thinning cuts to reshape the outer canopy of only those branches that are obstructing access or that risk causing property damage would be best during the summer. Leave the removal of larger branches and branch scaffold restructuring until winter when deciduous trees are dormant.

The recent overgrowth on many plants will have shaded the soil underneath the larger canopy as well as the roots that extended to the edge of the enlarged canopy. Those roots may well die back in the hot soil. Expect to see some wilting and signs of heat stress after even moderate pruning.        

The second caution on pruning overgrown plants this summer involves the risk of sunburn on newly exposed interior wood. We can expect that the Central Valley will experience several heat spikes, or long periods of high daytime temperatures (above 100 degrees) along with warm nighttime temperatures, in the next few months. Sunburn is a common problem on hedges that are pruned heavily during the summer in the Central Valley with the hedge tops showing most of the damage from the sun's rays. It's best to shear hedges lightly and frequently, every six to eight weeks, in the summer. To help reduce damage from sunburn, use a mixture of half water/half white latex paint on exposed wood, especially on citrus and other fruit and nut trees, to reflect some of the sun's harsh rays. The open center or vase-shaped pruning system on most fruit and nut trees should be maintained in the summer months.

The UC ANR publication 8057 on pruning deciduous trees has line drawings of several pruning methods and a glossary of pruning terms, which provide very basic pruning information. A long time favorite pruning guide, Orthos' ‘All About Pruning,' is once again out of print but worth searching for in used book stores for its excellent detailed descriptions, drawings and photos of the proper pruning techniques specific to many tree and bush species. DK Publishing (one of the best publishers of garden books) has “Grow Pruning and Training" ISBN 9780744026832, $12.99, which seems to be a good replacement for the Ortho guide. 

Choices for potting soil mixes have really expanded the last few years. In addition to the standard all-purpose potting soil mixes for acid- or shade-loving plants that have been available for many years local nurseries and especially the big box garden centers, these outlets now stock another dozen or more specialized types of potting soil mixes.

The terms ‘potting soils' and ‘potting mixes' seem to be interchangeable. There is no garden-type soil in these products. They are soil-less mixes of ground redwood or other tree barks, peat moss, composts and/or humus, sand and perlite and/or vermiculite. Vermiculite retains water; perlite improves drainage and aerates the soil to allow roots to spread more easily.

Big box garden centers are the primary sources of gardening supplies and equipment for the majority of home gardeners, especially beginner home gardeners.  It's worth taking a tour of a local big box garden center soils aisle to make note of the choices available and to check labels for specific information on potting soil mix contents and formulations. Master Gardeners' anecdotal information on the results of their use of preferred brands is extremely helpful as well.

The big box home improvement center closest to my home carries several nationally known brands of potting soil mixes, both organic and non-organic, for indoor plants, African violets, cacti and succulents, raised beds and container plants. Types and quality of components and the percentages of those components contained in potting soils vary widely by manufacturers; labels often provide minimal information.    

Some labels on both organic and non-organic potting mixes claim that their potting soils will feed plants for 6 or 8 months, and that less watering is required. The tiny list on the very bottom of the bag (reading glasses required) provides percentages of the additional fertilizer added to the soil. There may also be mention of an added wetting agent, but the wetting agent is not always named. Wetting agents or surfactants, both synthetic and organic (e.g. yucca), help keep potting soils from drying out and help water move through the soil. 

The percentages listed of added nutrients in almost all of the products is very low, 0.10 percent on average. A good quality low-nitrogen organic fertilizer will contain at least 4 percent nitrogen. The minute amounts of added chemically derived fertilizers can be easily leached out with watering. Organic potting soil mixes that contain natural ingredients like bat guano, earthworm castings or dehydrated manure may continue to provide lower-percentage nutrients for a longer period of time, but plants potted in either type of mix will require additional applications of a good quality fertilizer on a consistent basis to ensure vigorous growth and health throughout their growing seasons or lives. 

Most perennial potted plants benefit from annual repotting and soil replacement since potting soil mixes often quickly lose their capacity to retain water and nutrients during our long, hot summers, with or without an added wetting agent.  The time span between waterings depends mostly on the quality and composition of ingredients and the density and texture of the potting soil.