Why is it hard to find information on gardening in clay soils?

If you have been gardening in clay soil for very long you have probably had your fair share of failures. It is hard to find lists of plants that will thrive in heavy soil, as no one really wants to stick their neck out with recommendations in case it doesn’t work. Of course, there are many variables that need to be considered, and growing conditions are never identical, so please consider this a list of “plants to try” for clay soils.

What can be done to improve clay soils?

Clay soils lack two things: air and organic matter. A coarse compost, or even pine bark mulch is an important amendment to clay soils because it breaks up the small particle size of clay and attracts earthworms, which aerate the soil as they feed on the decaying organic matter.

One recommendation is to mix a 4” layer of compost into the soil 8”-10” deep, then work a 2” layer of expanded shale into the soil 6”-8” deep. While these amendments may help some clay soils, there may be some soils that will form a “perched water table”. This means that the water will completely fill up the amended layer to the point of saturation, THEN it will begin to penetrate the clay soil beneath. As you can imagine, this can be a problem with excessive rainfall or over-irrigation when the amended layer stays too wet and there is a lack of oxygen for the roots. Performing a Drainage Test to a depth of two feet may help you understand how your soil will perform under these conditions.

Berms are an excellent option for clay soils if they are built properly

In some cases, berms will be required to provide adequate drainage for your plants. Berms should consist of a Landscape Soil that contains topsoil and compost. Most importantly, berms should extend out five feet in all directions for every one foot in height and should taper down to the existing grade. Tall narrow berms dry out too fast and do not provide a wide enough area for support roots to grow to keep the plant upright. I have seen MANY failures with tall narrow berms!

Mulch placed on top of the soil maintains moisture and adds organic matter to the soil as it decomposes, making the soil less likely to compact. Mulch should be refreshed annually to replace that which has decomposed. Do not allow mulch to touch the trunk or stems of plants as it can affect the vascular system just under the bark.

How do you irrigate clay soil?

Irrigation is usually most successful when using the “cycle and soak” method, as it can increase the absorption depth in clay soils. Clay soils are more likely to “pool” water on top of the soil, as the percolation rate is slowed due to small particle size. Clay soils are often difficult to wet after they have dried out, which is why applying smaller amounts spaced hours apart can be more efficient.

If there is a slope, the water will “run off” of clay soil after the top few inches have absorbed what it can, and water will not penetrate any deeper in the soil. Cycling the irrigation with a “resting time” (usually a minimum of two hours) between the additions of water gives the soil time to finish absorbing the first addition of water. Watch the irrigation run and stop it when the water starts to “run off”. This will avoid wasting water that will not be absorbed if all the water is applied at one time. After a resting time, it will then likely absorb additional moisture. Adding compost breaks up the small spaces and helps with internal drainage, enabling the soil to absorb moisture more efficiently.

What are some ways to check my soil to see if it is clay?

Some helpful ways to check your soil to see if it drains well is to perform a Drainage Test. (Did I mention that before? It must be important!)

If you make a ball out of moist soil in the palm of your hand you can test it by sticking your finger into the ball. If your finger makes a hole in the soil ball, the soil is likely clay and will benefit from additions of compost. If the ball falls apart when you poke it with your finger, the soil is likely to have less clay particles and will probably pass a drainage test. These are just some helpful ways to get some information about your soil.

As I said before, I am hesitant to post lists of plants to be used in certain conditions because there are so many variables at play that cannot be considered.  Please use this list as a GUIDE and realize that every situation is different. Hopefully you will have some successes and not too many failures.

What are some plants that you can try in your clay soil?

Shade Trees

Bald Cypress                             Taxodium distichum

Bur Oak                                      Quercus macrocarpa

Cedar Elm                                  Ulmus crassifolia

Chinese Pistache                     Pistacia chinensis

Eastern Red Cedar                 Juniperus virginiana

Lacebark Elm                           Ulmus parviflora

Mulberry                                    Morus rubra

Pecan                                           Carya illinoinensis

Mex. Sycamore                        Platanus Mexicana

 

Ornamental Trees

Anacacho Orchid Tree       Bauhinia congesta

Japanese Black Pine           Pinus thunbergii

Mexican Plum                       Prunus mexicana

Texas Redbud                        Cercis canadensis var. texensis

Yaupon Holly                         Ilex vomitoria

Weeping Yaupon Holly      Ilex vomitoria var. texensis

 

 

Shrubs

Dwarf Yaupon                           Ilex vomitoria nana

Elaeagnus                                   Elaeagnus spp.

Elderberry                                  Sambucus spp.

Flowering Quince                    Chaenomeles speciosa

Nandina                                       Nandina spp.

Red Yucca                                   Hesperaloe spp.

Roses                                            Rosa spp.

Spirea                                           Spirea spp.

 

Perennials

Bee Balm                                     Monarda spp.

Butterfly weed                          Asclepias tuberosa

Calylophus                                 Calylophus spp.

Canna Lily                                  Canna spp.

Coneflower                                Echinacea purpurea

Coreopsis                                   Coreopsis spp.

Daylily                                         Hemerocallis spp.   

Esperanza, Sparklette           Tecoma spp.                 

Fall Aster                                   Symphyotrichum oblongifolium

Fall Obedient Plant                Physostegia virginiana

Fern Autumn                            Dryopteris erythrosora

Gregg’s Blue Mistflower       Conoclinium greggii

Hosta                                           Hosta spp.

Inland Sea Oats                       Chasmanthium

Iris German                              Iris germanica

Iris Louisiana                           Iris spp.

Lantana                                       Lantana spp.

Liatris                                          Liatris spp.

Meadow Sage                            Salvia nemorosa

Mexican Bush Sage                 Salvia leucantha

Mexican Honeysuckle           Justicia spicagera

Mexican Oregano                    Poliomintha longifolia

Mint                                              Mentha spp

Oxeye Daisy                               Leucanthemum vulgare

Perennial Hibiscus                 Hibiscus spp.

Rock Rose                                  Pavonia lasiopetala

Rosemary                                   Salvia rosmarinus

Rudbeckia                                  Rudbeckia spp.

Ruellia                                         Rudbeckia spp.

Salvia greggii                            Salvia greggii

Shasta Daisy                             Leucanthemum x superbum

Texas betony                            Stachys coccinea

Texas Star Hibiscus              Hibiscus coccineus

Turk’s Cap                                Malvaviscus arboreus var. drummondii

Umbrella Grass                      Cyperus alternifolius

Zexmenia                                   Wedelia acapulcensis var. hispida

 

Ground covers

Ajuga                                            Ajuga spp.

Creeping Jenny                        Lysimachia nummularia

Frog fruit                                    Phyla nodiflora

Horseherb                                  Calyptocarpus vialis

Mondo Grass                             Ophiopogon japonicus

Pigeonberry                               Rivina humulis

Sedum                                           Sedum spp.

Sedges                                           Carex spp.

 

Bulbs

Crinum Lily                                Crinum asiaticum

Oxblood Lily                              Rhodophiala bifida

Spider Lily                                   Lycoris radiata