A Case of Too Much Nitrogen

Why do fruit trees have dark green leaves but bear little or no fruit, especially if they are well fertilized? 

Here is a brief lesson in plant nutrition: in general, nitrogen is for vegetative growth. Consequently, when fertilizing a lawn or any plant grown primarily for its leafy growth, the main nutrient to apply is nitrogen. When growing plants for fruit or flower production, as opposed to foliage, a balanced complement of phosphate and potassium needs to be applied along with the nitrogen. Applying large quantities of a high nitrogen fertilizer to fruiting trees like citrus and avocado will put them in a ‘foliar growth mode’, producing an abundance of dark green leaves but little or no fruit. Be careful how much high nitrogen fertilizer you apply to the plant.

Urea fertilizer contains 46% nitrogen, ammonium sulfate 21%, while manures in general contain less than 5% nitrogen, usually around 2%. For more precise instructions on fertilizing fruit trees, CTAHR has a short publication titled Fertilizers for Fruit Trees in the Home Garden #37. It can be accessed online or at UH CTAHR Office at 875 Komohana St, Hilo, Hawaii. Other state extension offices in the U.S will have similar publications.

Deciduous Fruit Trees

 Hawaii, as well as other tropical climate zones, is not the best area to grow deciduous fruit trees.  These are trees like apple, peach, plum, pear, etc., that lose their leaves and go dormant in the winter. They actually require a dormant cold period in order to produce an abundance of good quality fruit. Deciduous fruit trees are rated by their chilling requirement, that is, how many hours below 45 degrees Fahrenheit they need to experience before breaking dormancy and yielding fruit.  Many trees like cherries and pears require 700 - 1200 hours below 45 degrees F during the winter.

  

Tropical areas do not have those conditions except to some degree at higher elevations.  Over the years, however, breeders have developed a number of deciduous fruit tree varieties with a low chilling requirement – 300, 200, or even as little as 100 hours of chilling. These are the varieties that are possible for Hawaii and other warm climate locations. (Yields and quality will not be equal to the better deciduous growing regions like California’s Central Valley.) In general, yields will be low, the fruit small, and foliage scant.   

When shopping for a deciduous fruit tree, select the low chill varieties: Apple – Beverly Hills, Anna and Gordon; Peach – Tropic Sweet, Florida Grande and Tropic Sweet. Normally, deciduous fruit trees are sold in January and February and are brought to the Hawaiian Islands from California.

Why is a Rain Tree Called a Rain Tree?

Botanically speaking, the tree is called Samanea saman, also known as the rain tree or monkeypod tree. There are a number of reasons why it is called the rain tree. 

• First, the leaves (actually leaflets) on this tree are light-sensitive and close together on cloudy days and at night.  This allows rain to fall through the canopy with little resistance. 

• Second, as certain sap-sucking insects like aphids feed, they also excrete sugary waste called honeydew.  If the tree is infested with a large population of these insects, the honeydew they produce will constantly rain down like a fine mist.  Don’t park your car under the tree while this is going on. 

•  Third, the nectaries on the leaf stems at times will also expel a sugary juice.  

       

The Beautiful Vireya

Vireya Rhododendrons are part of the rhododendron family (actually classified as a subgenus). They are native to Southeastern Asia (New Guinea, Borneo, Sumatra and the Philippines), growing quite often in the cooler mountainous areas. On the Big Island of Hawaii, vireya have been successfully growing from approximately 3,000 feet elevation to sea level.  There are about 300 species of vireya, also called tropical rhododendrons. Azaleas are part of this group.

Vireya grow well in the sun.  Yet locations which produce high light and temperature intensity in the afternoon should be avoided. In this case filtered sun is best. On the other hand, planting vireya in the shade will produce leggy shrubs with inferior flower production. The best advice is no hot summer sun and no heavy shade.

Planting vireya in soils and potting mixes with excellent drainage is important. In addition to vireya growing in the soil, they can also be found growing as epiphytes. An epiphyte is a plant that grows upon another plant, such as a tree, but is not a parasite on that tree and derives its moisture and nutrients from the air and rain and sometimes from debris accumulating around it.

Vireya are well adapted for growing in pots and may even grow well indoors in a well-lit area. They come in a variety of beautiful, vibrant colors such as salmon, pinks, orange and yellows.

For those interested in learning more about vireya, join the Hawaii Chapter of the American Rhododendron Society or search for a local chapter in other areas.