Extension

Cover Crops to Build Garden Soil

Shelby Mendoza, Extension Assistant

 

If you don’t plan to use your vegetable garden this fall, now is a great time to plan for planting cover crops. Cover crops can seem intimidating if you’ve never tried them before because there are so many options and a lot of information online. Really, though, it comes down to finding a mix that works for your situation—your location, your soil’s needs, and when you plan to plant your next round of vegetables.

 

Why use cover crops in a vegetable garden?

The benefits depend on which species you choose, but in general, cover crops can:

• Reduce soil erosion
• Suppress weeds
• Improve soil fertility by adding nutrients back into the soil

 

Good fall/winter cover crop options for Oklahoma gardeners

For most situations, a mix of a grass and a legume is recommended. Together, they suppress weeds, build soil organic matter (which improves soil structure, water infiltration, and nutrient retention), and add nitrogen to the soil.

 

Grasses

 

Hard Red Winter Wheat

• Erosion control
• Suppresses weeds
• Scavenges nutrients, bringing them higher into the soil profile
• Adds biomass that becomes organic matter when incorporated
• Easy to turn in with a broad fork or pitchfork
• Should be killed 2–3 weeks before planting the next vegetable crop

Oats

• Suppresses weeds
• Prevents erosion
• Scavenges excess nutrients
• Adds biomass for organic matter
• Winterkills after a heavy frost

 

Legumes

Legumes fix nitrogen through a symbiotic relationship with Rhizobia bacteria in the soil. This relationship allows legumes to convert atmospheric nitrogen into a plant-available form. Some legumes require an inoculant to achieve optimal nitrogen fixation. Once you have chosen a legume cover crop, check whether an inoculant is recommended for that species.

 

Crimson Clover (Trifolium incarnatum)

• Adds nitrogen
• Prevents erosion
• A pollinator favorite in early spring
• Adds biomass for organic matter
   

Hairy Vetch (Vicia villosa)

 (Can reseed on its own)

• Adds nitrogen
• Suppresses weeds
• Prevents erosion
• Improves topsoil tilth for the following vegetable crop
   

Tilth is the physical condition of soil, determined by its structure and suitability for plant growth. Good tilth feels crumbly, allowing for root growth, water infiltration, and aeration.

 

Other Options

 

Daikon Radish (Raphanus sativus longipinnatus)

• Breaks up compacted soils
• Suppresses weeds
• Scavenges nutrients
• Winterkills, releasing nutrients it absorbed back into the soil

Terminating Cover Crops

1. Timing of Termination
   1. Cover crops should generally be terminated 2–4 weeks before planting your next vegetables.
   2. This gives time for residues to break down and prevents them from tying up nutrients that your vegetable crops will need.
2. Methods of Termination for Home Gardeners
   1. Cutting/Mowing – Cut plants at the base with shears, a mower, or even a weed eater. Leave the residue on the soil surface as mulch or incorporate it into the soil.
   2. Tilling/Incorporation – Turn crops under with a tiller, broad fork, or spade. This helps residues decompose faster but disturbs soil structure more than leaving mulch.
   3. Winterkill – Some covers (like oats and daikon radish) naturally die after heavy frost, so no action is needed.
3. Things to Watch For
   1. Residue breakdown – High-carbon grasses (like wheat) take longer to decompose. Mixing them with legumes helps balance decomposition.
   2. Nutrient availability – Incorporating green cover crops too close to planting can cause temporary nutrient tie-up or cooler soil conditions, which may slow vegetable growth.
   3. Self-seeding – Some species (like hairy vetch) can reseed if not terminated before flowering.

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Planting Garlic

Tyler Mason, Assistant Professor

 

Garlic (Allium sativum) is a cool-season crop that is grown as an annual from about mid-October to mid-July in Oklahoma. It performs best in well-drained medium textured soil with a pH range of 6.2-6.8. Be sure to plant cloves about 1” deep with a vertical orientation. About 750 to 1000 pounds of seed garlic is needed per acre. Delivering 100 pounds/acre of nitrogen broadcast preplant or via side-dress over two or more applications in the spring will support vegetative growth. Similarly, phosphorus and potassium preplant incorporated at 100 pounds/acre, each will support the crop during its lifecycle. Another important input is weed control. Our research team needed to cultivate the rows to remove weeds in the crop over three events. High weed pressure is known to reduce yields. The top performing garlic cultivars over our two-year study at the Cimarron Valley Research Station in Perkins, OK were ‘Inchellium Red’ (2,346 lbs./acre), ‘German Extra Hardy’ (2,447 lbs./acre), ‘Chesnock Red’ (2,475 lbs./acre) and ‘Romanian Red’ (2,779 lbs./acre). Some producers reduce overall costs of garlic production by saving and replanting 15-20% of their total marketable yield.

 

Overwintering Annuals

Many of our annuals or bedding plants we use in the landscape are subtropical to tropical perennials and will often continue to grow if you can move them indoors. At the botanic garden, we take cuttings of our favorite species a few weeks before predicted frosts and root them in the greenhouse over winter. Plants that have been in pots in the garden all summer are even easier to move indoors. In the protective environment of your living room, an annual that bloomed repeatedly throughout the summer may continue to do so in the house.

 

To grow annuals indoors, start them with plant cuttings or cut back potted plants and bring them inside. Here are some tips from the experts at the American Association of Nurserymen (AAN):

 

• Use plant cuttings to start such annuals as impatiens, coleus, and geraniums. Within one to two weeks, they’ll develop roots in vermiculite, sharp sand, or perlite.
• Choose cuttings two to six inches long making the final cut just below a node (where the leaves were attached). Strip bottom leaves where the stem will be inserted into the rooting medium. Facilitate rooting by dipping the premoistened stem one inch into a rooting powder.
• Pinch off flower buds so they won’t draw on the cutting’s energy during the time it takes to develop roots. Maintain moisture and humidity by putting a clear plastic bag over the pot. Make sure leaves do not contact the plastic, as this will cause them to rot.
• Keep the rooting medium moist, not soggy. Each day check on the cuttings and immediately remove any rotting leaves from inside the bag. As the cuttings develop roots, keep it in an area of the room that is bright but not in direct sunlight.
•  After one or two weeks, new roots will develop. Plant in four- to six-inch containers and protect from direct sun for up to three days. Then put potted plants in the brightest section.
• Once the potted plant is established, pinch off the stem tip – this encourages growth of lateral branches that fill out the plant and cause it to flower more prolifically.
• To treat container-grown plants, trim them back to help them grow indoors. Reduce height by four to six inches. Petunias, geraniums, impatiens, lantana, and coleus are some plants that respond well to trimming and do well indoors.
• Before bringing in annuals, check for insect and disease problems. Check with your local garden center or nursery for the best suggestions for treatment.
• Once it’s taken care of, put the plant in a bright area of a room. Windows facing south or west are good locations. Give the plant a good supply of plant food and soon you’ll see new leaves develop and flowers begin to form.
• Because light is critical to encouraging a plant to bloom indoors, sun-loving annuals may not respond well to environments that have little natural winter light. Marigolds, zinnias, and other kinds of flowering plants need a good supply of sun, while less intense light suits coleus, impatiens, and some fibrous begonias.

 

Plants for Poorly Drained Soils

When dealing with poorly drained soils, additions of organic matter address many of the problems faced with garden soils. But sometimes, we run into problem areas in the garden where our efforts have little impact on drainage. Areas where water often drains that are low, have poor soil, or sit at the end of a slope can be such problem spots. 

 

In many urban sites the upper eight inches or so of the soil are in very good condition from years of compost and organic matter inputs, but when we dig deeper, we find compacted clay subsoil. This subsoil has very poor drainage. Plants often struggle to survive in these locations.

 

Our options for managing these sites can be limited. We could excavate the soil to 18 to 24 inches and add improved topsoil, but that can come at a great expense and can sometimes create more problems, and if we have established trees and shrubs growing in the area that is out of the question. Sub-surface drainage pipe can be installed to help pull excess water away from problem areas. And in some cases, we can just build up by installing raised beds. We can also reduce some of the water problems through careful irrigation management, but that is only part of the solution.  

 

Another option, which is generally easier, is to plant plants that don’t mind wet feet. The following plants tolerate flooding for short periods of time (normally only a few days). Very few of these plants will tolerate long-term submersion.

 

Trees

• Bald cypress, Taxodium distichum;
• Pond cypress, Taxodium ascendens
• Black gum, Nyssa sylvatica
• Cedar elm, Ulmus crassifolia
• Red buckeye, Aesculus pavia
• River birch, Betula nigra
• Willow, Salix species

 

Shrubs

• Buttonbush, Cephalanthus occidentalis
• Chokeberry, Aronia arbutifolia
• Deciduous holly, Ilex decidua
• Dwarf Palmetto, Sabal minor
• Redtwig Dogwood, Cornus stolonifera
• Southern waxmyrtle, Myrica cerifera
• Summersweet, Clethra alnifolia
• Virginia sweetspire, Itea virginica
• Yaupon holly, Ilex vomitoria 

 

Grasses

• Eastern Gama Grass, Tripsacum dactyloides
• Feather reed grass, Calamagrostis arundinacea
• Indian Grass, Sorghastrum nutans
• Inland sea oats, Chasmanthium latifolium
• Prairie dropseed, Sporobolus heterolepis
• Sedges, Carex species
• Seep Muhly, Muhlenbergia reverchoni
• Sweet flag, Acorus calamusSwitchgrass, Panicum virgatum

 

Perennials

• Beebalm, Mondarda didyma
• Blue Star, Amsonia tabernaemontana
• Culver’s Root, Veronicastrum virginicum
• Indian Pink, Spigelia marilandica
• Iron weed, Vernonia lettermanii
• Joe-Pye Weed, Eutrochium purpureum
• Mexican Petunia, Ruellia spp.
• Milkweed, Asclepias spp.
• New England Aster, Aster novae-angeliae
• Obedient Plant, Physostegia spp.
• Rain Lily, Zephyranthes spp.
• Red Star Hibiscus, Hibiscus coccineus
• Rose Mallow, Hibiscus moscheuto
• Spider Lily, Hymenocallis liriosme
• Swamp Hibiscus, Hibiscus militaris or H. laevis
• Swamp Sunflower, Helianthus angustifolius
• Turk’s Cap, Malvaviscus arboreus var. drummondii

 

Trees Add Value to a Landscape

Trees can be an asset to the landscape when the right species have been planted, and it has been placed in the right spot, and fall is the best time to establish most shade and ornamental trees. 

 

Trees offer characteristics and attributes like shape, height, size, growth habit, color, and flowering and non-flowering features. Trees add to the year-round interest by providing some backbone to the overall look of the landscape. In addition to aesthetic qualities, trees add utilitarian value such as wind protection, shade, wildlife habitat, visual screening, and other benefits to homeowners. In the community they contribute to the atmosphere of peace of mind and relaxation.

 

The wrong tree in the wrong place though, could lead to failure, disappointment, and at times, costly repairs. A tree placed too close to a home, sidewalk or patio could result in damage from branches or roots. A tree that produces fruits if placed too close to a patio or driveway will be a nuisance and require constant cleanup. 

 

Small landscapes should avoid very large trees. Note, with many larger species there may be a dwarf version available, plant breeders have recognized our home landscapes are smaller than they used to be and have looked for and bred into many popular species smaller forms of these trees to fit into today’s urban landscape. Be sure to select a species that is well adapted to the region. A plant that is stressed due to an inability to adapt to the local climate will often be short lived. 

 

Below are some suggested tree species that grow well throughout most areas of Oklahoma.

 

Shade Trees (40-60’)

• Bald Cypress (Taxodium distichum)
• Bur Oak (Quercus macrocarpa)
• Caddo Maple (Acer saccharum ‘Caddo’)
• Chinkapin Oak (Quercus muehlenbergii)
• Ginkgo (Ginkgo biloba)
• Japanese Zelkova (Zelkova serrata)
• Kentucky Coffeetree (Gymnocladus dioicus)
• Shumard Oak (Quercus shumardii)
• Silver Linden (Tilia tomentosa)

 

Small Trees (20-40’)

• American Hornbeam (Carpinus caroliniana)
• Chinese Pistache (Pistacia chinensis)
• Cedar Elm (Ulmus crassifolia)
• Crapemyrtle (Lagerstroemia indica)
• Desert-willow (Chilopsis linearis)
• Escarpment Live Oak (Quercus fusiformis) (evergreen)
• European Hornbeam (Carpinus betulus)
• Fringetree (Chionanthus spp.)
• Goldenraintree (Koelreuteria paniculata)
• Hedge Maple (Acer campestre)
• Horstmann Atlas Cedar, (Cedrus atlantica ‘Horstmann’) (evergreen)
• Oklahoma Redbud (Cercis canadensis ssp. texensis ‘Oklahoma’)
• Persian Parrotia (Parrotia persica)
• Saucer and Star Magnolias (Magnolia spp.)
• Shantung Maple (Acer truncatum)
• Trident Maple (Acer buergerianum)
• Vitex, Chastetree (Vitex negundo)
• Washington Hawthorn (Crataegus phaenopyrum)

 

Preparing Landscape Trees for Winter

While this year has seen plenty of moisture so far, it is not uncommon to have relatively dry winters. Following a dry winter, we can sometimes see landscape plants struggle. So the fall and winter are equally important to a plant’s health, especially our trees, which represent a long-term investment and significant value to the overall landscape. 

 

Tree roots continue to grow throughout the winter and need moisture to survive. Dry, cold soil can be damaging to a plant’s roots. Moist soils hold more heat than dry soil; so, the potential of damage to plants’ roots during the winter increases if the soil is dry and cold. To avoid further damage to already stressed plants, it is important to send our plants into the fall and winter with good soil moisture.

 

Before we talk about how to water trees, it is important to understand how tree roots grow. Basically, trees produce two types of roots, large perennial roots, and smaller, short-lived absorbing roots. The large perennial roots provide anchorage for the tree, water and mineral conduction, and food and water storage. Perennial roots are woody and increase in size and grow horizontally. In fact, 90% of the tree’s roots are in the top 12 inches of soil.

 

The smaller, absorbing roots are only about 1/16 inch in diameter, make up the major portion of the roots surface area and are responsible for the absorption of water and minerals. These roots grow outward and predominantly upward from the large perennial roots toward the surface where minerals, water, and oxygen are generally abundant.

 

Both the larger roots and small roots occupy a large area consisting of at least the area under and out to the dripline (the outer edges of the tree’s branches) and often well beyond that, up to two to four times the height of the tree.

 

Knowing how tree roots grow helps us determine the best method to water our trees. First, because we know that most of the tree’s roots are within the top 12 inches of soil, deep watering to a depth of 12 inches below the soil surface is recommended. Saturate the soil around the tree at and within the “dripline” to disperse water down toward the roots; watering only at the base of a large, mature tree is not adequate. The objective is to water slowly, dispersing the flow of water to get the water deep down to the tree’s roots. Watering for short periods of time only encourages shallow rooting which can lead to more drought damage. Using irrigation methods that distribute water faster than it can be absorbed by the soil is only wasteful. Watering at ground level to avoid throwing water in the air is more efficient.

 

In addition to deep watering, there are some additional tree maintenance procedures that can significantly increase a tree’s chance of survival.

 

Mulch around your trees with 2 to 4 inches of organic mulch to reduce moisture loss. Use wood chips, shredded bark, leaves, or evergreen needles as mulch. While many homeowners prefer to have grass growing under their trees, maintaining mulch within the dripline of the tree is better for the tree and is more representative of its natural environment, the woods. Avoid the use of stone or rock near trees as this may increase air temperatures and moisture loss from leaves and stems. Keep mulch 6” from the trunk of the tree.

 

Do not fertilize a tree that is under drought stress. Salts in fertilizer may burn roots when there is not sufficient water. Fertilizers may also stimulate top growth resulting in too much leaf area on the plant for the root system to maintain during periods of limited soil moisture. If fertilizer is needed and soil moisture is adequate, avoid high nitrogen fertilizers late in the season; actually, fertilizers should not be applied much after early September to avoid a flush of leaf growth that may not acclimate before freezing weather.

 

Keep your trees healthy and pest free. Postpone any construction activities planned near your tree to reduce impact on the trees’ roots and avoid traffic as much as possible to reduce soil compaction. If your tree has any insect or disease problem that may be adding additional stress – treat them accordingly to reduce the overall stress to your trees.

 

Properly prune trees and shrubs during time of drought to improve structure, limb stability and to remove dead and weakened branches. Leaving broken, dead, insect-infested, or diseased branches can further weaken a tree during drought and set the tree up for deadly secondary insect and disease problems.

 

Many tree species are harmed by herbicides used in the lawn. Trees already stressed by drought can be harmed by a heavy application of herbicide in the root zone. Avoid using herbicides during stressful times or only do spot treatments if possible. Be sure to always read and follow label directions.

 

Following these guidelines will help preserve our trees, the most valuable assets to our landscapes, and will also meet guidelines for water conservation during drought periods.

 

(Sources: HLA-6404 Winter Protection for Landscape Plants; HLA-6408 Landscape Maintenance Schedule).

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September Pecan Management

Becky Carroll, Senior Extension Specialist

 

 

This time of year, pecans are continuing to fill. Keeping leaves on the tree as long as possible—through adequate irrigation and insect and disease management—is critical for both the current crop and next season’s production.

 

Pecan Drop

“Why are my pecans dropping now?” is a common question in late summer. Nut drop can result from several stress factors, including excessive crop load, drought, or even prolonged wet conditions. Other causes include pecan scab, insect feeding during the water stage, and self-pollination. Water split is another issue in some thin-shelled cultivars, where shell hardening and the start of dough development create internal pressure, causing the shell and shuck to split and the kernel to abort.

 

Tree Spacing

Before harvest, take time to evaluate tree spacing and consider marking trees for removal. Marking trunks of nonproductive trees with spray paint can help identify candidates for later removal. Overcrowded orchards or groves reduce tree health and yield. Spacing can be assessed in several ways:

1. Maintain at least 10–15 feet between tree canopies.
2. Ensure no more than 50% ground shade at high noon during the growing season.
3. Allow about 30 square feet of cross-sectional trunk area per acre.

 

Fact Sheet HLA-6208:  Improving Native Pecan Groves provides details on tree removal decisions, considering factors such as irregular bearing, small or poor-quality pecans, insect and disease susceptibility, late ripening, or harvest difficulty.

 

Insect Monitoring & Control

• Pecan Weevil: Continue monitoring traps for adult emergence. See EPP–7190: Monitoring Adult Weevil Populations in Pecan and Fruit Trees in Oklahoma for trapping details. After heavy rainfall, expect emergence to increase and be ready to spray as conditions allow. Monitor nut development to determine egg-laying timing by cultivar.
• Stink Bugs & Leaf-Footed Bugs: These insects can cause bitter kernel spots in the dough stage. They often move into orchards from nearby crops or weedy areas once those habitats are disturbed.
• Aphids: Check regularly for aphid infestations. Thresholds are 20–25 yellow pecan aphids per compound leaf, but only 2 black pecan aphids per leaf. If treatment is needed, consider combining aphid sprays with weevil sprays to prevent flare-ups.

 

Disease Control

Continue fungicide sprays if scab models indicate risk (see the Oklahoma Mesonet Pecan Scab Advisory). After shell hardening, scab usually affects appearance more than kernel quality unless it is severe.

 

Harvest Preparation

Prepare the orchard floor by mowing vegetation, smoothing hog damage or cattle tracks, and removing fallen branches. Condition soil if planning to establish cover crops.

 

Irrigation

Adequate water is essential during kernel filling to produce high-quality pecans. Without irrigation, kernels may be light and wafery. Aim for about two inches of water per week, supplementing rainfall as needed.

 

Cover Crops

September is an ideal month for planting legumes in orchards or groves. See HLA-6250: Use of Legumes in Pecan Orchards for details. A healthy legume stand can supply most if not all the orchard’s nitrogen needs while also attracting beneficial insects. Cool-season legumes are especially advantageous since they compete less with trees for moisture.

 

Soil Testing

Soil testing is recommended every 3–5 years in mature orchards or groves. Monitoring soil pH, phosphorus, and potassium help guide nutrient management. For sampling instructions, see PSS-2207: How to Get a Good Soil Sample. Collect 15–20 cores at 0–6 inches deep from representative areas, mix them thoroughly, and submit as a composite sample for the best results.