GARDENING : Growing Plants Takes Feat of Clay
Gardeners in Orange County curse our clay soil. Sticky and mushy when wet and hard as cement when dry, it doesn’t provide the most hospitable plant environment.
“It’s very challenging to grow plants in it,” said Steve Kawaratani, landscape manager of Laguna Nursery and a California certified nurseryman (CCN). “Clay soil is very compact. The soil particles are so tightly bound together that plant roots don’t get much air. The soil also tends to retain water and salt, which can lead to root rot. And it is very alkaline, which many plants can’t tolerate.”
If you plant in our dense clay soil without properly amending, your plants are likely to do poorly. “Most plants fail because of improper soil preparation, not disease or insects,” said Kawaratani.
“The more prepared your soil is before planting, the higher your root to shoot ratio,” said Kawaratani. “Good root growth leads to healthy plants with more flowers and vegetables and shinier, glossier, larger leaves.”
Before preparing your soil, it’s important to know what you’re working with. To determine this, test the soil or have it tested.
“Fill a coffee can with dirt from four locations in the yard and bring it to your local nursery. They’ll have a soil and plant lab test it for about $50,” said Kawaratani. “Or spend $15 to $16 on an easy to use testing kit which can also be found at your local nursery.”
Testing your soil is useful because it clues you in to a number of critical things, including the fertility of the soil. It also measures pH, which will tell you how acid or alkaline your soil is.
“Most plants thrive in a soil pH of about 5.5 to 6.5, with 7.0 being neutral,” said Kawaratani. “Because we receive most of our water from the Colorado River, which is highly alkaline, our soil tends to be about 7.9 to 8. When pH is this high, plants are likely to be stunted, especially acid-lovers like gardenias, camellias and azaleas.”
A test will also indicate the salt content of your ground, which is critical because salt can be deadly to plants. “Excess salt often burns root ends which creates a favorable environment for fungi and bacteria,” said Kawaratani. “The roots then seal off and can no longer uptake water and you’ve got a case of root rot.”
Amending your soil with the right ingredients before planting will lighten the soil and give you better drainage and air penetration. This will prevent root rot and encourage rapid, healthy plant growth. It will also adjust the pH, decrease the salt content and add nitrogen.
“The most efficient conditioner for clay soil is redwood compost,” said Steve Hollister, CCN, manager of the Armstrong Garden Center in Irvine. He recommends two to three bales per 100 square feet.
“This may seem like a lot of amendment, but it takes a great deal of conditioner to change soil structure,” he said.
“When preparing soil for a lawn, work the compost into the first eight inches,” advised Hollister. “With shrubs and trees, dig a hole 1 1/2 to two times the existing root ball. Then combine the soil you’ve removed with a comparable amount of redwood, mix the two together, and put it back in the hole with the plant,” said Hollister.
Redwood compost is a byproduct of the lumber industry and can be found at your local nursery for $8 to $9 a bag.
“When you buy redwood compost, or any other soil amendment, make certain that it is composted or nitrolized,” said Kawaratani. “If the compost isn’t partially decomposed, instead of breaking down minerals to give to the plant, the soil will break down the compost, and the plant will miss out on vitally needed nutrients, such as nitrogen.”
Planter mix is another commonly used amendment, although Kawaratani says that redwood compost is preferable because it lasts longer--up to six or seven years in the ground.
Other amendments that help clay soil include gypsum, which is calcium sulfate. This flushes out salts and lightens the soil by making the soil particles larger. Kawaratani recommends adding 150 to 200 pounds of gypsum per 1,000 square feet. To lower the pH, add soil sulfur or try gypsite, which is a combination of sulfur and gypsum.
Another helpful product is Ironite, which adds a combination of iron and sulfur. “Iron is also often deficient in alkaline soils,” said Hollister.
Before using any of these products, look closely at package directions and contents. “It’s best to use just one or two of these products so that you don’t get ingredient duplication,” he said. “For instance, you wouldn’t want to add Ironite and gypsite, because they both contain sulfur.”
Perlite is another amendment that helps break up the soil. This is a popcorn-like product that doesn’t decompose. It helps aerate the soil and holds soil particles apart. On its own it isn’t a sufficient soil conditioner and should be added in conjunction with redwood compost or planter mix.
There are a few products you should never add, including vermiculite, which tends to squash and hold water; peat moss, which also holds water; sand, which will create a cement-like mixture when combined with clay, and lime, which will make the soil even more alkaline.
When you amend, make sure to mix the conditioner into the soil, rather than just around the root ball of the plant.
“If you just put conditioner near the roots, the plant will begin to do poorly when it hits the clay soil because of the high alkalinity and salinity,” said Kawaratani. “Or the roots will retreat back into the soil mixture and begin growing in circles and avoid branching out. This is especially dangerous for trees, which may topple over during winds because their roots are concentrated in one small area.”
This is all good information before planting, but what do you do if an existing plant is ailing?
First, you must determine if the plants are suffering from root rot, said Kawaratani. “You can spot root rot in plants from the leaves. The excess water comes through in the leaves and they burn. If leaves wilt and are brown and yellowed despite adequate watering, then root rot is likely.”
To make sure that root rot is the problem, it is best to perform a simple test. Get a professional soil probe or any long metal tube and push it down into the soil near the plant. You want to remove dirt from near the root zone.
“Once extracted, give the soil a sniff test,” said Kawaratani. “If it smells like rotten eggs, then the roots are dying.”
To save the plant, he recommends punching a series of holes around the perimeter of the plant’s root zone at six-inch intervals. Make them a 1/2-inch in diameter and a foot deeper than the original planting hole and fill them with sand.
“Doing this will allow excess water to drain off the root ball,” he said.
Cutting down on watering frequency will also help.
Because properly preparing soil takes a great deal of amendments and time, you may want to add plants that like clay soil.
Some plants that grow well in clay soil and are available in local nurseries include:
* acacia
* alyssum perennial
* bamboo
* blue atlas cedar
* bog rosemary
* bottlebrush
* bougainvillea
* California lilac
* cape weed
* carob tree
* common hackberry
* common yarrow
* coreopsis
* coyote bush
* creeping mirror plant
* crimson spot rockrose
* dwarf plumbago
* fernleaf yarrow
* glossy abelia
* golden wattle
* hollyhock
* incense cedar
* lily-of-the-Nile
* little gem
* Little Sur manzanita
* madrone
* naked lady lily
* orchid tree
* peppermint tree
* purple leaf baileyana
* sea thrift
* Sierra blue ceanothus
* snow in summer
* strawberry tree
* tree of Heaven
* trumpet creeper
* weeping myall
* Western redbud
* white alder
* white rockrose
* woolly yarrow
