Genetics determine hydrangea's flower size

  • Article by: Nancy Rose
  • Contributing Writer
  • January 9, 2002 - 10:00 PM

QI have some hydrangeas that are at least 40 years old. I have fertilized the plants but the flowers are always small. What can I do to get bigger flowers?

AWhile soil conditions and fertility can affect flower quantity and size, genetic heritage is the biggest factor in the size of your hydrangea's blooms. I'm assuming that you have smooth hydrangea (Hydrangea arborescens), a shrub that's 3 to 4 feet tall with many upright stems, large dark green leaves and white flower clusters that bloom in mid-to late summer.

The actual species, meaning the plant as it grows in the wild, does not have especially showy flower clusters. They are about 3 to 5 inches across, dome shaped, and primarily have small (one-quarter inch in diameter) fertile florets, with perhaps a few of the larger (three-quarter inch), showier sterile florets scattered around the outer edge of the cluster. You may have that species growing, in which case the flowers will never be any larger or fuller.

Most smooth hydrangeas now used for landscaping are cultivars that have been selected for having flower clusters comprised almost entirely of the large, showy, sterile florets. This makes the clusters quite large (6 to10 inches across), and rounded. 'Grandiflora,' sometimes called 'hills of snow hydrangea,' is an older cultivar with 5-to 8-inch flower clusters. A newer cultivar is 'Annabelle,' selected for its sturdy stems and massive (8-to 10-inch) white flower clusters. If you want a smooth hydrangea with large flower clusters, you will have to replace your old plants with a cultivar like 'Annabelle.'

QIn an answer to a question about evergreens losing needles, you mentioned that blue spruce often develop diseases after about 15 to 20 years. We have several 25-year-old blue spruce that are losing needles from the middle of the plant. What are the diseases that blue spruce suffer from and what can be done about them?

AColorado blue spruce (Picea pungens var. glauca) is susceptible to several foliar diseases that can cause needle loss, death of branches and even death of the tree. It's important to know which, if any, disease your trees have, so to make a correct diagnosis you may want to consult a certified arborist or the University of Minnesota's Yard and Garden Line. If your trees are losing needles only from the innermost part of the tree, it might simply be normal needle shedding.

Cytospora canker is a fungal disease that is marked by dead branches scattered throughout the whole tree. The disease creates diamond-shape cankers that eventually girdle the branches, which cuts off water and nutrient movement leading to death of the branches. There are no fungicides recommended for treating cytospora canker. The best defense is to keep the tree in good health; adequate watering is especially important since the disease seems to be worse under drought conditions.

Another common disease is rhizosphaera needle cast. This disease affects the lowest branches first and works its way up the tree, instead of randomly affecting branches throughout the tree. Foliage at the very end of infected branches usually looks healthy, while needles on the rest of the branch start to turn brown and drop. The entire branch will be dead the next year. On close inspection you might see tiny black dots (they are fungal fruiting structures) on infected needles. Rhizosphaera needle cast can be controlled if it's caught early. Remove all dead or infected branches in early spring. Then apply the fungicide chlorothalonil when the new spruce needles are about half grown (around late May or early June in the Twin Cities area), then spray again in about three to four weeks. You might have to spray for two or more years to control the disease.

-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.

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