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Sunday, August 16, 2015

Hibiscus Blooming but Leaves Too Small

Q. My hibiscus plant, which I planted from a pot to my outdoor flower bed, is blooming like it should.  The problem is, that the leaves are not getting any bigger than 1 - 1½" long and ¾" wide.  The new leaves also only get to that size.  I water and fertilize if with Miracle Grow regularly, but that does not help.  Any suggestions?

A. The appearance of plants will be different when growing in different climate zones. Appearance can also be impacted by your management practices. Let's cover a few of these.
Red hibiscus growing on our farm, Moca Family Farm, in the Philippines
Climate and Microclimates. Plants growing under high light intensities will have a different appearance then plants growing under lower light intensities. The principal differences are in leaf size, color and thickness. 

Leaves growing under higher light intensities, provided they are getting enough water and nutrients, will be dark green, they will be smaller, thicker or tougher and develop a thicker waxy coating on the leaf surface. The same plant growing under lower light intensities will have larger and thinner leaves with a waxy coating that is not as thick. 

If light intensities get extremely high then we will see leaf discoloration, yellowing or bronzing, on some plants because the light intensity is actually damaging the leaves. If the same plant does not receive enough light then the plant will become “leggy” with large distances between the leaves and thin stems that will not support its own weight. The plant will become "floppy".

Our job as a manager of this plant is to find the right location in our landscape, the right microclimate, that will give it enough light so that it will flower and have an appearance close to what we expect. Because we are in a desert, Hibiscus might not look exactly the same as it does in semi tropical or tropical climates but we can approach that look if we are careful where we plant it.

Fertilizer. The selection of the right type of fertilizer and amount of fertilizer will, to a large degree, influence the type of growth we get from a plant. We know for instance that phosphorus fertilizers are very important for flowering, fruiting, root development and production of oils in plants. 

If not enough phosphorus is present for the plant to use it will impact this type of growth. If too much is present, it can also impact growth of plants by "messing up" the soil chemistry. If fertilizers are applied to the soil, we generally do not need to apply phosphorus frequently.

Nitrogen is a different animal. Nitrogen is important for developing dark green color in leaves and stems and for "pushing" new growth. It is important in producing good leaf size and in the number of leaves and supporting stems produced. 

The nitrogen in soils available to plants are in "salt" form. Applying too much nitrogen can result in leaf burning or scorching along the edges and even plant dieback or death if excessive amounts are applied. Unlike phosphorus, nitrogen is removed from the soil fairly quickly unless it is in organic form. 

The nitrogen will be depleted in 4 to 6 weeks through plant uptake and removal from the soil by the irrigation water and "evaporating" into the air. Organic nitrogen is removed much more slowly and gives the plants a "steady feeding" of nitrogen over a longer period of time.
In your case you want to make sure that nitrogen is applied regularly through the growing season to maintain dark green color and "push" new growth. Combined with moderate amounts of shade, nitrogen will encourage more leaves and larger leaves.

Soil. Organic matter such as compost mixed into the soil at the time of planting and applied at least annually to the surface of the soil surrounding the plant will encourage more leaf production and larger leaves on those plants which have it in their genetics to produce this kind of growth. I have seen this numerous times on a number of plants particularly in parts of the plants that are shaded such as lower leaves.

What do you need to do? Understand that if your hibiscus is in a very hot, bright location that this location will limit the plants ability to produce larger leaves. Moderate amounts of shade will encourage larger leaf development, particularly in a microclimate that gets morning sun but afternoon shade. 

Get some compost and dig it into the soil surrounding the plant as much as you can. Apply about 1 inch of compost to the soil surface after you are finished and thoroughly wet the soil deeply immediately after you apply. At least once a year apply compost to the soil surface surrounding the plant and water it in thoroughly. 

Apply phosphorus either to the soil or to the foliage as a foliar spray at least once a year after it is finished flowering for the season. Apply high nitrogen fertilizers to the soil surrounding the plant in early spring, early summer and late summer. Winter tender plants should not be fertilized with high nitrogen fertilizers after August 1.

The three overall factors that will determine the leaf size: 
  • what it's capable of producing genetically (if you know this plant can produce much larger leaves but it is not doing it), 
  • soil improvement, and 
  • application of nitrogen fertilizers.

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