On a bright and early summer's morning, with not too much dew present, have you ever noticed the beaded necklace of water drops surrounding each leaf of a young, rapidly growing Dahlia plant?
Obviously a system is active that causes a lot of water to be transported up to the leaves where evaporation will ordinarily remove it during the daytime. This evaporation is the final stage of the water movement system called Transpiration.
Why so much water? Why the waste? Why not just enough? The explanation lies in the rapid growth potential of the Dahlia which requires a certain mineral or fertilizer amount that is present in only small concentration, therefore much water must be processed to get the necessary dissolved nutrients. Don't be tempted to add extra fertilizer to 'help out'. If the solution is too concentrated it will burn the delicate root and leaf tissues.
Considerable water is also required for the manufacture of the large amount of cellulose needed for the structure of the stems and other cell parts. Photosynthesis creates sugars from water and carbon dioxide and then later reactions convert the sugar into cellulose and also into the starch that is stored in the tubers. A little more water is required simply to keep each cell full and turgid so as to support the leaf and stem structures. We have all seen a droopy Dahlia or two that has either not got enough water available or has not developed an adequate root system to supply the plant.
This last symptom is often caused by planting large tubers, so that the Dahlia, being well supplied with food, does not send out sufficient nutrient seeking roots and subsequently cannot get the water that a large plant requires. Nothing much can be expected from such a plant. Small tubers or plants from cuttings are hardly likely to show this flaw and many people will either not plant a large tuber at all or else they will cut much of it away.
The water requirements of the Dahlia are therefore rather large at times. The demand will depend on the total leaf area and also upon the requirements for growth, and once mature, especially growth of blooms.
I have tried to grow Dahlias in tubs, but the chore of watering a mature, blooming plant is much too demanding for me.
My compliments to those who are committed enough to do it. Maybe a drip irrigation system could ......?...... Hum, something to think about!
Later in the season the growth of tubers requires water also, but not in the massive amounts needed by rapid cellular growth.
In the wild, Dahlias have a dormant season in their mountainous range which is quite cool, but mainly quite dry. The tuber has evolved as a water supply storage to keep the plant alive over the dry season. When the rains come again, growth resumes.
This would suggest that to get good tubers you should gradually withhold water at the end of the season to mimic the trigger of the natural dormancy period. In our Northwest climate, if heavy rains set in, you may as well dig them up as soon as there is a break in the weather. If you don't the tubers will rot or become so water rich that prolonged curing is required before storage.
You might even lose some of next years eyes if they begin to sprout in what may seem to the Dahlia to be the returning rains of the ancestral spring!
Another factor to consider is that both the plant and the tubers are more susceptible to frost when the natural antifreeze action of the sap sugars is diluted by excess water content.
Overwatered or waterlogged conditions are never satisfactory as the acidic, low oxygen environment inhibits root growth and nutrients are not gathered and growth suffers.
Conclusion? Water Dahlias as little as you can when they are small in order to promote root growth to seek out the limited moisture. Then increasingly more water as leaf area becomes significant, and continue through the blooming season. Then less water to ripen the tubers and bring the season to a natural end. Common sense is required, of course, and if they look like they need a little water, then water them! For example, young plants or seedlings will often wilt badly on a very hot day and there is nothing wrong with a few light spays to raise the humidity and cool them off. I would caution you however not to soak them every day to try and prevent the wilting. Let them be stressed a bit and they will make better roots and be better plants.
The only time that water is essential, beyond that required to keep the plant looking good, is just as the buds are forming. At this stage it is essential to water thoroughly and often or the number of petals in the forming buds will be decreased and a much less desirable bloom will result.
Water for cut flowers
A lot of "garden lore" is based on observations that are loosely connected to the facts. Here is a perfect example.
Correct Observation: fresh tap water does not promote flower keeping quality.
Wrong Conclusion: It is the added Chlorine.
Fact: gases are much more soluble in cold water, especially under pressure, as in our tap systems. This causes Oxygen and Nitrogen, which make up 99% of air, to dissolve. Putting flowers into this water when cold is a mistake because as the water warms (conditions) the disolved gases form small bubbles IN THE STEM channels, blocking them. Letting the water stand and warm a bit allows this excess air to escape so that the stems can take up water unobstructed.
Many exhibitors add a drop of bleach (Chlorine) to their cut flower water to kill bacteria which can grow and block the water take up system. Adding a touch of sugar duplicates the typical cut flower extender formulas. No fertilizer is required.
On a related note. I feel that, while foliar spray does get absorbed by the
Completely dormant tubers will have cell sap with a small ionic component of essential salts that give a small antifreeze buffer. Once started into growth, the starch aboard is converted into sugar and the antifreeze effect increases quite a bit.
Copyright © 1996 , 1999, 2003 Wayne Holland
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