Sunday, January 2, 2011 7:40 AM HST
Do you remember the nursery rhyme, “Here we go round the mulberry bush, the mulberry bush, the mulberry bush. Here we go round the mulberry bush so early in the morning?”
That little ditty was popular around 1840, when mulberry trees were gaining popularity in Europe and on the U.S. mainland but had only recently been introduced to Hawaii. Sometime in the early 1800s, a black-fruited cultivar of Morus alba known as “nigrobacca” was introduced to the islands for landscape use. Today, the trees are known here as lovely, drought tolerant plants that produce edible, tasty berries.
The genus Morus is in the Moraceae family and includes about 15 botanically separate species, all known as mulberries. The classification is complicated with numerous cultivars and varieties that have resulted from widespread hybridization of the species. All are deciduous trees that are native to warm and subtropical regions, mostly in Asia.
Mulberry trees were cultivated for several thousand years in their native China as food for silkworms. The worms, whose cocoons produce silky thread, are very host- specific. Today, billions of pounds of mulberry leaves are consumed by silkworms throughout the world in the annual production of the more than 70 million pounds of silk. To date, silkworms are not known to exist in Hawaii, however.
Many attractive features beyond silk making have made mulberry trees popular. The leaves are large, dark green and broadly oval with serrated margins. Some varieties and younger trees also produce attractive leaves with two or three lobes. The spreading branches and distinctively furrowed bark of the tree add to its appeal. Although some varieties can grow to 40 feet over time, they remain small for many years and can be easily maintained with judicious pruning. The trees can grow in a variety of soil conditions and prefer full sun. Once established, they have limited water requirements and require little care beyond picking the delicious fruit.
As new mulberry leaves develop, usually in mid-spring, tiny male and female flowers appear on slender, inconspicuous spikes. The pendulous catkins grow at the axils of the current season’s growth and on spurs of older wood. They are mostly wind-pollinated, though some cultivars will set fruit without pollination.
Morus alba is known in the botanical community for the rapidity with which its flowers fire pollen into the air by rapidly releasing stored elastic energy in the stamens. It is the fastest known movement in the plant kingdom, in excess of half the speed of sound.
Mulberries are an aggregate fruit, composed of lots of berries, each with its own seed, stuck together in a long cylinder hanging from a short, slender fruit stalk. Some varieties produce clusters over an inch long. The fruit is usually white or green when immature, becoming red and finally turning dark purple to black with a sweet flavor when fully ripe. Unlike other fruit and berries, mulberries ripen over an extended period of time.
When you see fruit dropping or birds congregating, it’s time to harvest. Gathering the ripe fruit helps limit any invasive possibilities the birds might cause. Place a drop cloth under the tree and shake the limbs. Individual picking may be necessary with some varieties but should be done while wearing gloves and an apron because the berries have a red juice that stains hands and clothing. With this in mind, trees should be planted away from homes, sidewalks and driveways to prevent stains to cement or by having the berries tracked into the house on shoes.
Unwashed berries will keep several days in a refrigerator in a covered container or can be frozen for later use. They can be eaten out of hand or used in muffins, pies, tarts, puddings or sauces. Mulberries also make good wine and are excellent dried.
The mature plant, especially the bark, contains significant amounts of resveratrol, known for cancer prevention and longevity. All parts of the plant are marketed in various forms as nutritional supplements. The white sap in the unripe fruit and green parts of the plant is said to be intoxicating and mildly hallucinogenic.
Mulberries can be grown from seed, although seedlings can take up to 10 years to bear.
For this reason, mulberries are most often propagated from cuttings from producing trees. Placed in a rooting compound of half vermiculite and half perlite, the cuttings should root quickly. Once roots are established and new growth is happening, they are ready to plant out.
Mulberry trees are easy to prune to maintain size and shape but overaggressive pruning can be harmful. The trees can be kept compact by allowing a set of main branches to develop and then pruning laterals to six leaves midsummer so that spurs will develop near the main branches. It is not advisable to prune the trees heavily since the plant is inclined to bleed at the cuts. Cuts of more than 2 inches in diameter generally do not heal and should be avoided at all cost.
Mulberries are generally free of pests and disease, although cankers and dieback can occur and “popcorn disease” is an occasional problem. If fruits swell to resemble popped corn, collecting and burning infected fruits is advised.
Mulberry trees need little nutritional additions. Maintaining healthy mulch is usually enough but, if trees seem deficient, an annual addition of a balanced fertilizer can help.
If you want to plant your own mulberry trees you might want to try calling local nurseries like Aikane in Hawi who order and stock mulberry trees regularly. Once you have a tree or can find another grower who has some you can take cuttings from and produce new plants easily. Ask farmers at the Keauhou Farmers Market on Saturday. Many of them grow mulberries for their own use or to make jams and might be willing to sell you some cuttings.
Duff is a plant adviser, consultant and an organic farmer living in Captain Cook.