7.10.18 — Longer days and hotter temperatures mean fruit trees are in their active growing season. What are our fruit trees doing during the active growing season in a physiological sense? What do they need? Joanna Glovinsky, founder of LA’s Fruitstitute, a fruit tree care and education service for backyard growers, explains the Hows and Whys of summer fruit tree care.
Here at Fruitstitute our aim is to teach backyard growers how to understand their fruit trees or, as we put it, how to read your tree. Our approach is to take the science of fruit tree care out of the orchard and make it accessible to everyone who’s interested in growing great fruit in Southern California. Now that summer is here, the time is ripe to talk about how to ensure a sweet summer harvest.
HOW YOUR TREE MAKES FRUIT
To put it simply, the long days of summer mean your tree’s energy intake and output are at the highest. This is because increased daylight boosts photosynthesis, the process through which leaves convert energy from the sun into carbohydrates, which kicks off the metabolic processes of plants. (Remember that high school science class?) I’ll break this down even further so you can get the full picture.
The conversation starts with stomata, an outer layer of cells on a leaf responsible for photosynthesis. These cells are like millions of little mouths that open to feed during the day and close at night. In general, the more hours the sun hits the leaves on a tree, the more hours stomate stay open. As stomata open, water inside the leaf, which traveled up from the soil, evaporates. This physiological process is called transpiration. The water evaporating from each leaf on a tree creates a suction effect throughout the tree that pulls water and soluble nutrients from the soil up through the roots and into the rest of the tree, a process called translocation. The longer the day, the longer the stomata are open, the longer transpiration and translocation occur. One thing worth noting is that stomata close when a tree needs to conserve water. So, on really hot days stomata will not be open as long and the tree does not lose more water than it can handle. Similarly, if a tree is underwatered, the stomata will not open.
Open stomata are also consuming energy from the sun (photosynthesis), which is then converted into carbohydrates, the energy the tree needs to grow and make its various parts. This is a very simplified description of the process called respiration. Chlorophyll, the stuff that makes leaves green, is the secret ingredient here. So, the longer the day, the greater the rate of respiration.
Taking the above into account, we can see that the leaves of a tree – their size, color, access to sunlight – are critical for the tree to carry out these physiological processes that allow for growth. We can also see that for these physiological processes to occur optimally, a tree needs sufficient water and sufficient soil nutrients.
HEALTHY TREES MAKE TASTIER FRUIT
Let’s put these pieces together and a picture of what a healthy fruit tree looks like in the summer should become clear.
A healthy tree has:
-A healthy canopy of foliage. Not too dense but not too thin, green in color throughout and leaves that are generally clean of debris and grime
-Good soil fertility
If any or all of these three things are off, the health, rate of growth and fruit-bearing capacity of the tree are compromised. Moreover, the greater the degree to which any of these three things are off, the greater the tree’s health is compromised. That’s because without the right amount of photosynthesis, water and nutrients, your tree cannot properly produce or budget its resources. The result of which will ultimately lead to a sad tree with sad fruit.
For deciduous trees, those that lose their leaves and go dormant in winter, summer is particularly important. All the energy they create in summer is then stored during winter and used to create next year’s growth. Should the tree not produce enough leaves for example, or should these leaves be lacking in chlorophyll year after year after year, the tree will eventually fail (i.e. die).
THE LABOR OF SUMMER FRUIT
What does all this mean in terms of summer tree care? First, it implies that you should make sure your trees are being properly watered. Second, you need to consider your soil fertility. If you didn’t do it in the spring, make sure you apply your growing season soil amendments – compost/fertilizer and woody mulch – as soon as you can this summer season. If you see discoloration in leaves, you likely have a nutrition deficiency and should amend your soil.
That takes care of our feeding our roots. But what about our leaves? To answer that question, we first need to understand what your tree is growing in the summer. This is the same as saying, how does a tree allocate its energy resources in summer.
In summer, your tree allocates most of its chemical energy toward shoot and fruit growth. Root growth is reduced as a result, which is partly why summer is not the best season to plant a tree. New shoots are developing and as they do, they’re growing the flower buds for next season. For flower buds to form, leaves on these shoots need to photosynthesize sufficient amounts of chemical energy to make the stuff that forms flower buds. These new shoots have also added a new layer of foliage to your canopy that may be crowding or shading other branches. Too much shade and/or crowding means fruit on the affected branches cannot ripen as it should and the leaves cannot photosynthesize as they should. What do you do? The third component of summer tree care is summer pruning.
Summer pruning is all about optimizing light penetration throughout your canopy. When pruning your fruit tree, thin branches that have grown too tall and are shading/crowding the canopy and clean out all dead wood and debris. Because temperatures are hotter, bug populations are more prolific, so you don’t want to create too many wounds for bugs to enter with your pruning cuts. Summer pruning should remove only what is necessary. Similarly, don’t remove any bigger branches in the summer either. Doing so invites bugs and diseases. You could also wash down your canopy with water if you notice extra grime built up on the leaves, which hosts bugs. As always, know how to make proper pruning cuts before garnishing your blades.
Finally, to the fruit, the other thing your tree is allocating its energy toward growing in summer. Picture a skinny branch overloaded with fruit. If you leave all that fruit on that branch, the fruit may ripen but that branch only has so much energy to give to each fruit. However, if you were to thin some of that fruit, and leave one fruit per every few inches or so, depending on the size of the branch, that branch has that much more energy to allocate to each one of those fruits. Accordingly, the fourth and sweetest component to summer fruit tree care is fruit thinning. Fruit thinning not only makes your fruit tastier it’s also so important for the health of young trees, overbearing trees and for any branch being weighed down by the weight of its fruit. Wouldn’t you rather have a few superior fruits than a lot of inferior ones? You gotta thin it to win it.
By Joanna Glovinsky