Gardening : ‘Super Sponges’ Keep Plants From Drying Out : Polymers: When these man-made compounds are mixed with soil, they soak up water, then release it to the plant roots.
Much of the water given to plants goes right past them in a process called “percolation.”
Gravity grabs hold and pulls the water down past the roots. A certain amount of this is a good idea because the water carries away salts and other substances that accumulate in the soil and could become harmful to plants, although a good, soaking rain usually does this job.
Most of the time, however, water that percolates down through the NEW WAYS WITH WATER
One in a series of articles on water-saving plants, techniques and technologies.
soil is simply wasted in the garden--although it does find its way to underground reservoirs in time.
To try to prevent this waste--to trap water in the soil--organic amendments are often added to the soil at planting time. They hang onto the water, like little sponges, and make it available to the roots between waterings.
While experts now wonder if these organic amendments make that much of a difference when planting permanent things such as trees or shrubs in clay or good loam soils--which naturally hold onto water, there are new, Space Age amendments that hold much more water than other amendments.
Called “super sponges” by some, they are man-made polymers that can absorb as much as 150 times their own weight in water, when mixed with soil or a potting media.
A polymer is a chemical compound “consisting essentially of repeating structural units,” according to the dictionary, and under an electron microscope these new polymers look like a bunch of packing boxes joined together to form a maze of water-storing cells.
Add water and they swell up considerably (enough to pop a plant out of a pot if you use too much), and they store this water until the plant’s roots take it out. Roots quickly find the polymers and actually grow into them.
In the garden, this stored water is like an insurance policy, or water in the bank. If you forget to irrigate, don’t water on time or water erratically, these little reservoirs are there for the roots to draw upon.
Public agencies, including Caltrans and a number of Southern California cities, have found that the survival rate of newly planted trees, shrubs and even lawns goes up considerably when polymers are added to the soil at planting time.
At the Los Angeles County Arboretum in Arcadia, polymers are being used in the backfill that goes into the planting holes for trees and shrubs.
Bill Raymond, public works superintendent for the city of Cypress, reports similar results--better survival of new plants, faster initial growth and less water used. He has found that they only have to water half as often as before on areas where polymers have been added to the soil.
Home gardeners have discovered that plants that like more water than others, such as fuchsias, delphiniums and even roses, do better when planted with polymers because they have more water to draw upon, at least for a few years.
Container gardeners are the most excited however, because they don’t have to water nearly as often. In pots, polymers retain water without impeding drainage. Some premium potting soils will soon contain polymers because their usefulness is well established.
In all of these cases, water that formerly ran down through the soil--past the roots--or out of the bottom of the pot, was instead stored in the polymers, at least some of it.
This is how polymers save water--you aren’t really giving the plants less water, it just lasts longer in the soil because it is entrapped. It doesn’t percolate down, it doesn’t evaporate up.
Home gardeners will find that Broadleaf P4 Water Storing Granules is the most widely available polymer.
It is a particularly long-lasting, sophisticated, cross-linked acrylamide copolymer that works for five to 10 years, depending on the soil. It eventually breaks down into harmless elements and is considered as such by the EPA. It is quite expensive but a little goes a long way.
How can you put polymers to work in your garden?
You can’t just scatter them around or even till them in around existing plants. They must be down in the soil where the roots are and they are most useful and cost-effective at planting time in soils that do not naturally hold water well--such as sandy or gritty soils--or in potting soils.
A natural clay soil, even a good loam soil, is also a great storehouse for water. But a polymer can greatly aid a plant that has not yet grown roots out into the surrounding soil. And, during this time of establishment when watering must frequent, polymers can save a lot.
To ensure the survival and quick growth of new plants, put polymers in the planting hole.
At the Arboretum, they mix the polymer in its dry form with the excavated soil in a wheelbarrow, then put it back in the hole. Try to concentrate the polymers near the bottom of the hole to force roots to grow down.
The admirably complete instructions that come with Broadleaf P4 suggest using a mere tablespoon mixed into the backfill when planting something from a gallon nursery can; one-half cup for a plant in a five-gallon can. Planting holes should be twice the size of the container or root ball.
Bare root roses get particular benefit from polymers. In an article in “The American Rose” magazine, Thomas Cairns suggests adding about a half cup to the planting hole for new roses. Roses are thirsty plants. The P4 instruction sheet even tells how to make a gelatinous slurry in your blender that the bare roots can be dipped into.
Although they do not take the place of organic amendments in vegetable gardens and flower beds (organic amendments add fertilizer elements and improve the workability of a soil), polymers can help flowers and vegetables grow faster and stronger with less frequent watering. C. Dean Piper, a soil scientist at Broadleaf, suggests using 3 pounds. of P4 for every 100 square feet of soil, mixed into the top six inches of soil.
Certain especially thirsty plants love having a little polymer, even a pinch or two, placed under them at planting time. Try a little under delphiniums, or fuchsias, or tuberous begonias for some surprising results. In my own garden, delphiniums that were normally 4-5 feet tall grew to 6-7.
Caltrans, among others, has found that polymers give new sod lawns a real boost. Here, the recommended rate is 25 to 30 pounds per 1,000 square feet. P4 is available in 10-, 25- and 55-pound bags as well as in tiny little packets.
In containers, mix a tablespoon into two gallons of potting mix. The instructions suggest that the preferred way of mixing P4 is wet, after it has soaked up all the water it can. If you do not mix it in wet, do not fill containers more than three-fourth full or the soil will expand right out of the pot when the polymer does become wet.
In all cases, be sure to water areas where polymers have been added less often, 50% less, or half as often--but only after a couple of weeks pass, to allow the roots to search out the polymers.
Because the water is stored in the polymer below ground, it is a little difficult to judge when to water because the soil surface can appear quite dry; you must watch the plant for signs that’s its time to water again.
And, fertilize less often since fertilizer is also stored in the polymer’s cells.
Polymers would be less useful if we all knew how to water precisely, replacing only the water lost to evaporation, transpiration and percolation, but precise watering is a science and for most, polymers promise better plants with less water in pots or when first starting them in the garden.
Plants already in the ground are less likely candidates for polymers and mature trees and shrubs will derive little benefit. In these cases, more precise irrigation with drip systems or micro sprinklers is the way to save water and make healthier plants.
