Crown

In the world of plants, crown can mean two very different things.

Like the fancy hat on a monarch’s head, crown can refer to the canopy of a tree. It can also mean the part of a plant slightly above and below the soil line. In both cases, the more you know about them, the better your plants will grow.

​Tree top crown

Technically, the crown of a plant refers to everything that is above ground. Most people, however, use the term to describe the outer branches or canopy of a tree. In either case, mature crown size is an important factor when selecting a site for a tree. While most trees don’t mind mingling their branches, there are a few species that exhibit ‘crown shyness’ and will grow in such a way as to keep their distance from the branches of other trees. Tree crowns are classified by their shape. They can be rounded, weeping, funnel-shaped, spreading, pyramidical, oval, or conical.

David W. Lamb, et al (2016). Comparison of Canopy Volume Measurements. Remote Sensing, 8(5). Reproduced with permission of the authors.

Leaves that make up the crown are responsible for far more than just photosynthesis. In addition to being the major food manufacturing system of the tree, they also filter out dust and other particles from the air, slow the speed at which raindrops hit the ground, and shade the ground below the tree, stabilizing soil temperatures for the root system. [Seven or eight trees also produce the oxygen you need to breath each year.]

Tree crowns can be reduced moderately using heading cuts. Pruning in this way can lead to increased stem development lower in the tree, which means even more pruning to maintain air flow and sun exposure, while limiting the fruit load to a level that the tree can safely support.

​Ground level crown

The ground level crown is where the shoots, stem, or trunk meets the root system. Very often, when a root stock is grafted to a fruiting stock, this is where the graft will occur. If you look closely, you should be able to see a grafting scar. Grafted or not, the crown is a vulnerable area. Fertilizer, mulch, and standing water should be kept away from the crown. It is much better for your tree’s health to irrigate and fertilize closer to the drip line. The drip line is the outer edge or silhouette of the overall tree.

Crown diseases

Being exposed to water and soil, the crown is susceptible to fungal infestation and other problems. Some of the more common pests and diseases of the crown area (and their hosts) include:

Diagram of a typical lawn grass plant with crown circled (Kelvinsong) CC BY 3.0

• bacterial crown rot - artichoke
• black rot (black crown) -  carrot
• botrytis crown rot - lettuce
• crown and root aphids - carrot, parsley, tulips
• crown rot - beets
• crown and spear rot - asparagus
• crown gall - all fruit and nut trees, especially stone fruits, and grapes
• fusarium crown and root rot - asparagus, summer squash
• phytophthora root and crown rot - all fruit and nut trees, eggplant, peppers, tomatoes
• raspberry cane borers - all cane fruits
• sclerotinia stem and crown rot (white mold) - alfalfa

In most cases, these diseases can be prevented with simple cultural practices:

• install plants at the proper planting depth; do not bury the graft union
• proper irrigation; avoid standing water
• keep mulch several inches away from trunks and stems
• ensure good drainage
• adjust sprinklers so that trunks are not hit
• use clean seeds and disease-free seedlings, transplants, and bare root trees

Exceptions to the rule

In some cases, transplants can be replanted deeply enough that the lowest set of leaves end up underground. These leaves should be removed at transplanting time. The nodes where the leaves were are then transformed into root tissue, increasing the availability of water and nutrients found in the soil. This practice is not recommended for most plants. However, tomatoes and peppers, in particular, can increase their yields substantially with this practice. I have heard mention of using the same technique on brassicas, such as cabbage and broccoli, but I could not find any verifiable proof, so I am skeptical until proven otherwise.
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As you walk through your garden, be sure to inspect the ground level crowns of your plants for signs of fungal disease and pests. Then, look skyward for a quick check on the overall form of your trees. These quick checks can reduce your workload and protect your plants over the long haul.