Phyllis asks: On my walks recently, I have noticed a plant I haven’t seen before. I think it is an invasive weed. Can you identify it and let folks know about its invasive potential?
Tropical Gardener answer: Thanks for noticing this very invasive plant and calling our attention to it.
From the photos that you sent, this looks like a plant in the Macaranga genus. It is probably Macaranga tanarius, known commonly as the parasol tree, or the perhaps the larger Macaranga mappa known as bingabing. Both are highly invasive members of the Euphorbiaceae family.
According to the Hawaii Pacific Weed Risk Assessment (HPWRA) scale both rank at 12 or very high invasive risk. This screening tool evaluates many factors that determine a plant’s invasive potential. The scale starts at 1 and plants that fall below 3 on the scale have a low invasive risk. Plants between 3 and 6 have invasive potential. Plants that rate above 6 on the scale qualify as invasive species. The higher the number the more invasive they are. UH has a good site with a table of plants and their invasive potential at http://www.botany.hawaii.edu/faculty/daehler/wra/full_table.asp.html.
Macaranga is native to southern India and China as well as northern Australia and some Pacific islands. Both species are popular as an ornamental tree in many places. The trees were first introduced to Hawaii as ornamentals in the mid-1920s. They tend to grow straight up on a slender trunk when young and can quickly grow to over 30 feet tall. They colonize rapidly forming dense stands which shade out other plants and threaten native species and their habitats here in Hawaii.
Public awareness of these and other invasives is a good way to help control their spread. The Big Island Association of Nurserymen in Hawaii has voluntarily agreed to not order or sell Macaranga tanarius or M. mappa to prevent further use of them in ornamental gardens. They agree that they should not be cultivated and the public is discouraged from planting them.
Their distinctive leaves often have light colored veins and are joined to the tree by stems of the same color. The stems attach to the leaf on the underside near the center. This attachment gives the leaves a slight bowl-like, umbellate appearance. The leaves tend to be large, rounded at the base and pointed at the tip.
The trees produce flowers fairly early in life. M. tanarius produces clusters of small cream colored flowers on long panicles. M. mappa’s flowers are pink and petal-less appearing in clusters near the base of the leaf stalk. The fruit that follows flowering is encapsulated in a yellow covering with soft spines. When ripe, the fruit turns black and is often eaten by birds.
Bird droppings are a major way that the seeds of this plant are dispersed.
Of course, the best thing we can do to prevent further spread of these Macaranga species is to not plant them. As the Hawaii Invasive Species mantra goes, “Don’t plant a pest.” It can also be helpful to look for them as you walk or drive around the island.
If you see them when they are small, you can pull them out by the roots. If you see them once they have grown to tree size, you can alert the property owners to their presence or ask for permission to remove them. If you see flowers or fruit, collect them and destroy them. Fruit needs to be ground up in a garbage disposal or subjected to another technique that will destroy their viability.
Keep looking for invasive plants and do your part to prevent their spread.
Email plant questions to email@example.com for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living part time in Kailua-Kona.Gardening Events
Tuesday: “Little Fire Ant Workshop” from 9-11:30 a.m. at the Kona Cooperative Extension Service in Kainalu across from the Aloha Theatre. Registration is required. To register or get more information, contact Gina at 322-4892 prior to class.
Saturday: “Work Day at Amy Greenwell Garden” from 9 a.m. to 12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers will be able to help with garden maintenance and are invited to bring a brown bag lunch. Water and snacks provided. Call Peter at 323-3318 for more information.
Farmer Direct Markets
Wednesday: “Ho’oulu Farmers Market” 9 a.m. to 2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay
Saturday: “Keauhou Farmers Market” 8 a.m. to noon at Keauhou Shopping Center
“Kamuela Farmer’s Market” from 7:30 a.m. to 1 p.m. at Pukalani Stables
“11Waimea Town Market” from 7:30 a.m. to noon at the Parker School in central Waimea
“Waimea Homestead Farmers Market” from 7 a.m. to noon next to Thelma Parker Gym in front of Thelma Parker Library.
Sunday: “Pure Kona Green Market” 9 a.m. – 2 p.m. at Amy Greenwell Garden in Captain Cook
“Hamakua Harvest” 9 a.m. to 2 p.m. at Hwy 19 and Mamane Street in Honokaa
Plant Advice Lines
Anytime: firstname.lastname@example.org Tuesdays &Thursdays: 9 a.m. to noon at UH-CES in Kainaliu – 322-4892
Mon., Tues. &Fri: 9 a.m. to noon at UH CES at Komohana in Hilo 981-5199 or email@example.com