Horticulture Tips

March 2021

Oklahoma Cooperative Extension Service

Division of Agricultural Sciences and Natural Resources

Department of Horticulture & Landscape Architecture

Oklahoma State University


David Hillock, Consumer Horticulturist

Lawn and Turf

  • Remove excessive thatch from warm-season lawns. Dethatching, if necessary, should precede crabgrass control treatment. (HLA-6604)
  • Broadleaf weeds can easily be controlled in cool-season lawns at this time with post-emergent broadleaf herbicides.
  • Preemergent crabgrass control chemicals can still be applied to cool- and warm-season turfgrasses. Heed label cautions when using any weed killers near or in the root zone of desirable plantings.
  • March is the second-best time of the year to seed cool-season turfgrass; however, fall is the best time to plant. (HLA-6419)
  • Cool-season lawns such as bluegrass, fescue, and ryegrass may be fertilized now with the first application of the season. Usually, four applications of fertilizer are required per year, in March, May, October, and November. (HLA-6420)
  • Begin mowing cool-season grasses at 1½ to 3½ inches high. (HLA-6420)

Flowers & Vegetables

  • Cultivate annual flower and vegetable planting beds to destroy winter weeds.
  • Apply mulch to control weeds in beds. Landscape fabric barrier can reduce the amount of mulch but can dry out and prevent water penetration. Thus, organic litter makes the best mulch.
  • Prune roses just before growth starts and begin a regular disease spray program as the foliage appears on susceptible varieties. (HLA-6403 & EPP-7607)
  • Avoid excessive walking and working in the garden when foliage and soils are wet.
  • Start warm-season vegetable transplants indoors.
  • Divide and replant overcrowded, summer and fall blooming perennials. Mow or cut back old liriope and other ornamental grasses before new growth begins.
  • Your cool-season vegetables like broccoli, cabbage, carrot, lettuce, onion, peas, spinach, turnips etc. should be planted by the middle of March.
  • Watch for cutworms that girdle newly planted vegetables during the first few weeks of establishment.  Cabbage looper and cabbageworm insects should be monitored and controlled in the garden. (EPP-7313)

Trees & Shrubs

  • Prune spring flowering plants, if needed, immediately following their bloom period.
  • Plant evergreen shrubs, balled and burlapped, and bare root trees and shrubs.
  • Anthracnose control on sycamore, maple, and oak should begin at bud swell. (EPP-7634).
  • Diplodia Pine Tip blight control on pines begins at bud swell.
  • Chemical and physical control of galls (swellings) on stems of trees should begin now. (EPP‑7168 & EPP-7306)
  • Dormant oil can still be applied to control mites, galls, overwintering aphids, etc. (EPP-7306)
  • The first generation of Nantucket Pine Tip Moth appears at this time. Begin pesticide applications in late March. (EPP-7306)
  • Control Eastern tent caterpillars as soon as the critters appear.


  • Continue to plant strawberries, asparagus, and other small fruit crops this month.
  • Start your routine fruit tree spray schedule prior to bud break. (EPP-7319).
  • Remove winter mulch from strawberries in early March. (HLA-6214)

Pecan Topics for March Zoom

Becky Carroll, Associate Extension Specialist, Fruit and Pecans

March 12 at 1pm is the second installment for the Pecan Topics zoom series for 2021. The meeting will cover timely topics for pecan growers and homeowners. Subjects on the agenda include Dr. Mulder discussing control of phylloxera. Charles Rohla with Noble Research Institute will cover fertilization and how to apply properly. Sprayer calibration will be highlighted during the session as well.

Advance registration is required and available at this link -  https://dasnr.zoom.us/meeting/register/tJAvdu-qqTMjEtb5Sh2vRYuw86WUN4obOlmX

The program is offered to anyone and at no charge. Extension educators who participate will receive in-service credit.  Please feel free to promote to your pecan audience.

Information and recordings of previous sessions are available on the Oklahoma Pecan Management webpage- http://okpecans.okstate.edu or the Oklahoma Pecan Management Facebook page - @okpecans.

Questions can be emailed to becky.carroll@okstate.edu.

Spring Cleaning

David Hillock

If you haven’t cut back your ornamental grasses and perennials by now, this would be a good time to finish this spring cleaning chore. New growth will begin to emerge soon on some grasses and perennials; waiting until new growth is several inches high will make it difficult to remove dead foliage without damaging the new growth. In addition, old leaves may be harboring diseases and insects from last season that could infect new growth if not removed from the garden. Removing old leaves also allows plenty of sunlight in to warm the soil and encourage new growth.

Pruning Roses

David Hillock

Rose plants need pruning to tidy up their appearance; control size; and improve their vigor, growing habits and bloom. Pruning methods vary according to the type of rose plant. To keep them in bounds, spring pruning usually is more drastic. Prune about 3 to 4 weeks before the average date of the last killing frost in your area. In most of Oklahoma that would be around the 15th of March. An exception to this rule involves climbing roses, which need to be pruned after flowering in early spring.

Probably no other aspect of growing roses has aroused as many questions as has the subject of when and how to prune roses. By following a few simple rules, you can improve their appearance and vigor and control the quality and quantity of the flowers. Some fundamental practices of pruning roses correctly in all gardens, regardless of type, are: 1) remove any canes that have been damaged by insects, diseases or storms; 2) remove one of two canes which may be rubbing one another; and 3) remove canes that are spindly or smaller in diameter than the size of a pencil. After pruning, according to these general recommendations, cut hybrid teas, floribundas, grandifloras and polyanthas back to 12 inches for large flowers and 18 to 24 inches for many smaller sized flowers.

Climbing roses generally are pruned to renew plant vigor by removing the old canes since the most productive and finest blooms on climbers are produced on canes that arise from the bottom of the plant the previous year. These newer canes produce more desirable growth and flowers. Since the canes may become quite long, it is necessary to prune them back so they are maintained in the desirable area.

Old fashion or antique roses require much less pruning than modern roses. Left unpruned old fashion roses will naturally obtain a rounded shrub shape. Pruning of these roses should be confined to some shaping of the plant, removal of damaged branches, and judicious trimming back to encourage growth.

On all roses, consider the cutting of the flowers as a form of pruning. When gathering roses, always leave at least two sets of leaves on the branch from which you cut the flower to insure plant vigor. When removing faded, spent flowers, cut only as far as the first five-leaflet leaf. Make cuts on the ends of branches at 45-degree angles just slightly above an outside facing bud with the lowest point on the side opposite the bud, but not below the bud itself. Never leave stubs when removing branches, since these die and can cause problems on the plant later. Always remove branches by cutting to a lateral branch or bud, or back to the base of the rose plant.

For more information on growing roses in Oklahoma see fact sheet HLA-6403 Roses in Oklahoma.

Dividing Hosta

Casey Hentges, Oklahoma Gardening Host and Laura Payne, Oklahoma Gardening Field Producer

The general rule about dividing herbaceous perennials – if it blooms in the fall, divide in the spring; if it blooms in the spring, divide in the fall; and if it blooms in the summer, divide in either spring or fall, but fall is preferable. 

So, using this rule, hostas are summer bloomers so division can be done in the spring or fall.  Fall is preferred because the hosta will be going dormant soon and transplanted into a new location will allow new roots to establish the plant as soon as the weather is appropriate.  Dividing in the spring means a newly divided plant will be transplanted to a new location, with no established root system and therefore may not be as well prepared for the heat of summer. 

However, dividing in the fall means dealing with the foliage still on the plant.  Therefore, it is easier to divide in the spring before the leaves emerge.  Another reason to divide in the spring is, when buying a new plant, if it’s large enough, it can be divided to make stretch the budget. 

Hosta, can be divided using a handsaw, an old serrated kitchen knife or a spade. To alleviate some of the stress on the plant, water thoroughly prior to dividing.  Also, if the hosta was in the ground, watering ahead of time will make it easier to dig. When digging a hosta from the garden, start digging about 6-18 inches away from the clump to get as much of the root ball as possible.  Once the plant is dug up, remove any excess dirt to show the growing points, cut between growing points. Ensure each new division is still a nice sized plant.  The hosta division should have adequate root volume and 2-3 growing points at the crown of the plant.  After division, these new plants can be relocated to a new location and should be planted in a hole at the same depth as they were prior.  The divisions can then be planted individually throughout your shade garden.  Planting a cluster of three division that are still spaced appropriately will create a large clump impact without the need to have a massive plant which may need dividing soon.  Be sure newly divided plants are well-watered for the first two weeks, especially if there is a period of drought. Most hostas grow several years without needing to be divided; however, if the plant starts declining, this may be a sign that it’s time to divide.  

Establishing a New Vegetable Garden

David Hillock

The past year has been a real challenge for a lot of people, but if anything good has come of it, there has been a surge in gardening activity and a desire to be outdoors. And why not? Gardening and being out in nature can be so rewarding and has proven to be therapeutic. If you are considering gardening for the first time or just want to get better at it, here is some information that will help get you started.

Site Selection

The following is a list of considerations when selecting a site for the vegetable garden:

  • Sun exposure: select a site that receives at least 6 hours of direct sunlight each day. Southern exposures are ideal for greatest sun incidence.
  • Soil: Well-drained soils such as sandy loam provides ideal conditions for growing vegetables. Soil pH near 6.6 is optimal. Avoid steep slopes where erosion will be a problem.
  • Air flow: avoid low-lying areas as these tend to collect cold air which slows germination and plant development in spring.
  • Avoid placing a vegetable garden near walnut trees. Walnuts exude a substance called juglone from their roots which is allelopathic, meaning it can kill other plants. Tomatoes and other solanaceous plants are highly sensitive to juglone.
  • Make sure the site is situated near a water supply.

Removing Vegetation

It is important to start with a clean slate when preparing a new garden bed. And this means removing existing vegetation and controlling weeds. Usually, this is a chore for the summer prior to planting. There are several methods available to kill off vegetation. The most common method is to apply an herbicide, but there are other non-chemical methods such as solarization and smothering.

Solarization is a simple technique that captures radiant heat energy from the sun and uses that heat to kill seedlings and weed seeds, as well as some soil-borne disease organisms. Sheets of plastic are used to trap the solar heat. Solarization is commonly used to kill weed seeds in areas where the vegetative layer has been removed.

To smother weeds, cover the soil with black plastic, or several layers of newspaper. Carpet or boards have also been used for smothering.

Solarization can be combined with other control methods. For example, an herbicide may be used to make the initial kill, then solarize to control subsequent seedlings and kill seeds in the soil. Solarization can also be combined with the application of soil amendments and fertilizers. In fact, solarization can speed up decomposition of organic matter, releasing soluble nutrients into the soil.

Whatever method is used, it is ideal to control perennial weeds before establishing a new garden. It will be much easier to manage them before you have the area planted with vegetables.

Soil preparation

Once the vegetation is removed, till the soil to loosen it. This is a good time to add manure or other organic material. To preserve soil structure, avoid tilling when the soil is too wet. To determine if the soil is too moist for tilling, grab a handful of soil and squeeze it slightly. If it sticks together in a ball it is too wet. If it crumbles easily it is ready.

How to Collect Soil for Testing

Soil tests should be included as part of garden preparation. It is easier to amend soils and add nutrients before planting, rather than after. Soil tests collect information on soil nutrients and pH.

When collecting soil samples, test areas with drastically different soil conditions separately. To get started you will need a tool for collecting small samples. A soil probe is a great tool for sampling if you have one. A shovel or even a small bulb planter can also be used. You will also need a bucket for sampling. You should obtain a representative sample for each area being tested. To do this, collect several samples from across the entire area being sampled and combine them into a single, representative sample. Take samples to a depth of six inches. In a large garden, as many as 15 to 20 cores should be taken.

Make sure to use a clean bucket that does not have any cleansers in it. Many cleaners contain chemicals that could alter your soil test results. Mix samples taken from one area together, then fill the sample bag for analysis.

Sample bags are available at your county extension office, where soil samples may also be submitted. The samples are sent to the OSU Soil, Water, and Forage Analytical Laboratory for testing. Tests cost $10 each, and evaluate soil pH, nitrate nitrogen, phosphorous and potassium contents. You can also request micronutrient tests as well as organic matter content and other specific tests. Test results include fertilizer recommendations specific to the type of vegetation growing on the site. Be sure to mark the proper space on the sample label indicating the type of area sampled, such as turf or garden.

Extension L-249 contains detailed information on collecting soil samples.

Don’t forget that even if you don’t have a large space for an in-ground garden, container gardening can also be done successfully. For information about growing in containers see our fact sheet HLA-6458 Container Gardening.

Irrigation System Maintenance: Spring Start Up

David Hillock

Now is a good time to prepare your irrigation system for the season. Before turning it on make a visual inspection of the sprinkler heads. Check for broken heads or covered up heads; free heads, make height adjustments, and be sure spray heads are still properly orientated. Check all valve boxes for rodent nests and debris.

Make sure there is power to the controller and set stations for proper run times. Turn the main water source on slowly to fill the system. If you have manual drain valves, leave them open to allow air to escape as the pipes fill with water; when water starts coming out the drain valves, close them.

Turn on each irrigation zone one at a time or set your controller to run through each zone using a test cycle setting. If choosing to run a test cycle of each zone, set a time limit long enough to observe each zone and mark needed repairs, about three minutes.

While each zone is running, walk through the yard and check each sprinkler head, noting any that require attention. Flag or mark problems to make them easier to identify when making repairs; look for leaks, make sure all heads are providing adequate coverage to their area and are closing properly. If the system is not running properly, additional troubleshooting should begin and repairs made. If major issues are discovered, an irrigation specialist may be needed to fix the problems. Replacing backup batteries could also be done at this time.

Irrigation technology has come a long way over the past several years so if your system is old and not as efficient, now would be a good time to consider upgrading the system. Smart controllers, such as climate-based controllers and soil moisture sensor controllers, provide easier access and more precise control of the system. Add-ons that also make systems more efficient are soil moisture sensors, rain and freeze sensors, and wind sensors. Smart sprinkler heads that provide better coverage without waste are also available.

Making sure the system is running properly and efficiently now will ensure your landscape plants will be healthy going into the growing season.

Chilling Hour Update for 2021

Becky Carroll

Take a look at February 2021 Hort tips for last month’s comparisons.

The following maps show the number of hours accumulated as of January 29, 2021 and February 25, 2021.  Notice differences in the two maps - the blues indicate higher chilling with the green show lower chilling.  In those areas with higher numbers of chilling hours, the trees are likely in the second stage of dormancy and ready to begin growth when temperatures warm. This may mean an earlier bud break or bloom for some fruit trees. It will be interesting to see how the map changes this next month. The panhandle is lagging in hours but most of the state has gained enough chilling to likely satisfy most tree and small fruit crops requirements. Let me know when you start seeing buds swelling on fruit crops.  

I will continue to update the map in the next issue of Horticulture Tips to see how this season progresses. These maps are not available on Mesonet but if you’d like more information, please contact me. 

2021 Oklahoma Proven Plant Selections

David Hillock

Each year a set of plants is chosen by horticulturists that will help consumers choose plants appropriate for Oklahoma gardens. The program began in 1999 by selecting a tree, shrub, perennial and annual worthy of Oklahoma landscapes. Now in its 23rd year, there are many plants to choose from. Selections for 2021 are listed below. To see all the plants recommended by the Oklahoma Proven Plant Selection Program, visit our web site at http://oklahomaproven.org/.

Tree – Magnolia grandiflora ‘Southern Charm’, Teddybear® Southern Magnolia

Southern magnolias are the southern belles of the evergreen plant world. The species can reach 80 feet high and 50 feet wide producing large, fragrant, creamy white flowers and traditionally have been found on large estates and plantations of the south, however, these are too big for many urban landscapes today. We have a solution for that though, enter ‘Southern Charm’ also known as Teddybear®.

Teddybear is a dwarf, compact version of the species. It grows about 16 to 20 feet high and 10 to 12 feet wide in an upright pyramidal form. This nice, tight growth habit makes it suitable for smaller gardens, screens, avenues, and specimens, as well as growing in large planters and containers. It is not too finicky about soil types, but grows best in deep, nutrient-rich, acidic, well-drained soil. Water deeply and regularly in first few growing seasons to establish an extensive root system. Once established, it grows best with regular moisture, but will tolerate brief periods of drought. Feed in early spring before new growth emerges. It grows best in full sun and benefits from organic mulch.

Leaves of Teddybear® are deep green and glossy above with a dense reddish-brown fur on the underside, thus the Teddybear® name. Flowers of Teddybear® are large, up to 8 inches across, saucer-shaped, white, and fragrant that appear in early summer until early fall.

No serious insect or disease problems.

  • Exposure: Full sun to part shade
  • Soil: Moist, well-drained
  • Hardiness: USDA Zone 7-9

Shrub – Itea virginica, Virginia Sweetspire

Virginia sweetspire is a native shrub to eastern Oklahoma but does well in many other areas of the state. It is a mound-shaped, slender-branched, deciduous shrub generally 3 to 6 feet tall and wide. Small, white, fragrant flowers bloom in spring to early summer in 4-inch spires that droop with the arching branches. Flowers open from base to tip so that the plant appears to bloom for a long time. Leaves turn red to purple in fall and persist well into the winter.

Plants are found growing in the wild in moist, even wet to swampy areas and along stream banks, in acid soils, but they are not too picky of the soil type. Plants should be watered during droughts. Virginia sweetspire grows in shady areas as an understory plant, but it grows best and has better blooms and fall color if it receives full sun for at least part of the day.

The long tassels of white spring flowers and red to orange and gold fall foliage make this an attractive ornamental. It is most effective when planted in masses, as single plants tend to be scraggly. It can be used as an understory plant, for erosion control, in a rain garden, and as a nectar plant and has no serious insect or disease problems.

Several improved cultivars exist such as ‘Henry’s Garnet’, which grows 2 to 3 feet high and 4 to 6 feet wide, has larger flowers and reddish-purple fall color. Little Henry® (‘Sprich’) is a dwarf cultivar growing approximately 3 to 4 feet high and wide with 3 to 4-inch flower spikes and burgundy-red fall color. Scarlet Beauty™ (‘Morton’) is upright-rounded to 3 to 4 feet high and wide with abundant flowers and orange-red fall color, and Scentlandia® is purported to be the most fragrant with large flowers, excellent fall color and grows only 2 to 3 feet high and wide.

  • Exposure: Part shade to full sun
  • Soil: Moist, acid soils, though tolerant of variety of soils; tolerates poor drainage
  • Hardiness: USDA Zones 5-9

Perennial – Anemone hupehensis var. japonica ‘Prinz Heinrich’, Prinz Heinrich Japanese Anemone

Prinz Heinrich Japanese anemone is an excellent perennial for late summer to early fall color that will grow in full sun, but in Oklahoma it is best planted in a part shade location or spot protected from the late afternoon sun and winds. It prefers fertile, consistently moist soil that is neutral to slightly alkaline with good drainage. In full sun and dry conditions, the foliage will often become burned; avoid wet soils, particularly in winter. In too much shade the flower stems tend to flop.

Foliage of Prinz Heinrich is dark green, softly pubescent beneath, and 3-parted on long petioles providing nice texture throughout the growing season. Flowers are semi-double, rose-pink, with narrow overlapping tepals surrounding a central cluster of golden-yellow stamens. Flowers are produced on long, upright, wiry but graceful branching flower stems that arise well above the basal foliage. The plant grows to about 28 inches high and spreads by rhizomes.

Prinz Heinrich can be planted in perennial borders, woodland areas, and are best grouped together in masses. It is a great plant for pollinators and attracts butterflies to the garden. It has no serious insect or disease problems though occasional pests can occur.

  • Exposure: Sun to part shade
  • Soil: Moist, well-drained soils
  • Hardiness: USDA Zone 5-8

Annual – Cuphea (species and cultivars)

Cuphea is a genus of about 260 plants native to the warm temperate and tropical regions of the Americas. Depending on the species and cultivar, they go by several common names such as firecracker plant, cigar flower, Mexican-heather, bat flower, bunny ears, candy corn plant, and false heather. Cuphea is a tender perennial grown as an annual in Oklahoma. It is low maintenance and continues to gain popularity throughout the country particularly for its tolerance to heat and drought. Plant foliage is bright green to blue-green and typically glossy. Although the flowers are small, they are abundant and provide a spectacular show all summer long with no need to deadhead. Plants come in a variety of forms with a variety of flower colors and forms that are vivid and produce a sweet nectar that attracts hummingbirds, butterflies, and other pollinators. Breeders have produced more compact plants with showier flowers that fit just about anywhere in the landscape or garden and they can be used as a houseplant.

Some of the more popular types include firecracker plant (Cuphea ignea), Mexican-heather (C. hyssopifolia), bat-faced cuphea (C. llavea), and candy corn plant (C. micropetala) along with many hybrids. Flowers of the firecracker types are tubular, bright red with black and white lips giving the appearance of a lit cigar. Mexican-heather is a nice compact plant that grows 1 to 2 feet high and about as wide with horizontal branching, bright green leaves, and bright lavender, trumpet shaped flowers. Bat-faced types (also called tiny mice or bunny ears) grow as rounded, bushy plants to 20 to 30 inches high and wide. Flowers are tubular with a purple calyx and a pair of red ear-like petals that resemble the face of a bat. Candy corn plant is considered the most cold-hardy of the cupheas and grows 1 to 3 feet high and wide producing yellow and orange bicolored flowers resembling the candy we enjoy in the fall. Cultivars and forms of these species offer flower colors from warm and spicy reds and oranges to cool pink, lavender, and purple tones. Cultivars with pure white blooms are also available.

  • Exposure: Full sun
  • Soil: Moist, well-drained soil; tolerates drier soil after establishment
  • Hardiness: Use as an annual