Wild garlic control in nursery crops

As the season changes so do the spectrum of weeds in our fields. Winter annuals and cool season perennials emerge as the summer annuals wither and fade. There are many winter weeds that can be problematic in nursery fields; however, the weed that rules the winter landscape is a bulbous perennial. Wild garlic (Allium vineale) is one of the most troublesome weeds in nursery production. This article will describe the biology and life cycle of wild garlic, and how to use that information to develop an effective control program.

Wild garlic is a cool season perennial in the family Liliaceae. It grows in small grass-like clumps from late fall through early spring. Wild garlic is problematic in wheat crops because its aerial bulblets (explained below) are difficult to separate from wheat grains. Contamination of this type can ruin the flavor of bread and other products made from wheat. It has also been suggested that cows that eat wild garlic have off-flavored milk.

So why is it a problem in nursery crops? We don't eat nursery crops. Furthermore, because wild garlic has a small and shallow root system, it does not compete with crops for nutrients and water. The major issue with wild garlic in nursery production is that our customers do not want this weed in their fields or landscapes. Shipping nursery stock with wild garlic bulbs in the rootball will likely cause problems with your customers, who will not appreciate the 'bonus' plants. This ultimately reduces the perception of quality in your nursery’s product.

Wild garlic structure

Wild garlic leaves are hollow and circular in cross-section. Wild onion (Allium canadense) is often confused with this plant, however, its leaves are flat and solid. Wild onion is also fairly easy to control and does not persist in cultivated crops, so its occurrence is not likely in maintained nursery stock.

Wild garlic plants can be divided into two main categories, scapigerous and non-scapigerous plants. Scapigerous plants are those that produce a flowering scape (a leafless peduncle). Non-scapigerous plants are those that do not.

Wild garlic reproduces by seed, aerial bulblets, and underground bulbs. Seed are thought to be a relatively minor pathway for plant dispersal. Seed are viable when produced. However, flowering scapes (and thus seed) occur on only a small percentage of plants in each field. Furthermore, scapes don’t develop until early summer (May to June) and they grow to a conspicuous height of approximately 2 feet. Thus it is unlikely that scapes avoid mowing, hoeing, cultivating, herbicide applications, and all the other forms of weed control that occur throughout the spring and early summer.

Aerial bulblets also form at the terminal end of scapes, often just below flowers. A single plant can produce from 20 to 300 aerial bulblets. Developing bulblets are enclosed in a spathe, which is a dry, thin, membranous bract that surrounds the bulblets until maturity. In other agricultural crops, aerial bulblets are commonly thought to be the primary mechanism by which this plant spreads. More aerial bulblets are produced on a plant than underground bulbs. However, flowering scapes are not likely to go unnoticed nor uncontrolled in a decently maintained nursery field. So just like flowers and seeds that also occur on the flowering scapes, aerial bulblets are probably not important in the spread of this plant throughout nursery fields.

There are several types of underground bulbs produced by wild garlic including offset bulbs, central bulbs, and hardshell bulbs. Offset bulbs are formed by scapigerous plants only. One offset bulb is produced for each scapigerous plant, in which the bulb develops just to the side of the scape (hence the name). Central bulbs form only in non-scapigerous plants and are central to the main axis. Both bulb types germinate in fall and produce several new hardshell bulbs the following spring.

Just like domesticated garlic, hardshell bulbs (you might call them cloves in garden lingo) are produced immediately adjacent to the original bulb. Plants normally produce 1 to 6 hardshell bulbs per season. New hardshell bulbs are fully formed in early spring. Hardshell bulbs can sprout the following fall, however, only 20 to 40% of hardshell bulbs germinate the first year while the rest lie dormant up to 6 years.

Wild garlic does not have a spreading root system, and thus does not spread rapidly throughout fields. Dig up a clump of wild garlic foliage and you will find an assortment of bulbs and shallow roots. The primary mechanism by which hardshell bulbs are dispersed throughout a production field is via tillage equipment during the summer.

Wild garlic life cycle

Research from other parts of the country has documented wild garlic’s life cycle. Clumps of foliage emerge from bulbs, bulblets, or seed in September. Plants continue to grow vegetatively throughout late fall, winter, and early spring. Additional underground bulbs are formed in early spring (March). Seed and aerial bulblets are formed on flowering scapes in May and June, then plants die and wither away by mid-June.

To make more effective control recommendations for Oregon nurserymen, it was necessary to determine a precise date at which wild garlic produces new bulbs in the spring. Working with a local nursery, we dug wild garlic plants every two weeks for an entire winter and examined the progress of new bulb formation. We determined that new bulb formation in Oregon's Willamette Valley occurs mid-March. In Figure 4 notice the bulb swelling to one side. By exerting pressure with my index finger, the immature bulblet was detached from the mother bulb. Undisturbed, it may have taken several more weeks for this new bulb to fully mature and release from the mother bulb. Nonetheless, at this stage herbicide applications may be too late to prevent the next generation of bulbs from forming.

New bulbs are in the final stages of development at the same time red maple (Acer rubrum) is flowering. Herbicide applications should occur prior to this, with a final application no later than when red maple begins blooming. Use established landscape trees as indicator plants instead of nursery stock. Trees still in nursery production can flower at odd times due to disrupted biological clocks in the quasi-natural ecosystems of nurseries.

The use of red maples to predict the production of wild garlic bulbs is an example of a phenological indicator. Phenological indicators are common temperature-dependent biological events (plant flowering, insect emergence, bud break, etc.) that are used to predict another event such as wild garlic bulb formation. Phenological indicators are useful and reliable tools for timing farm operations.

Wild garlic control

Kill wild garlic plants throughout fall, winter and early spring before plants can generate the next generation of bulbs in March.

Hoeing throughout winter or early spring will prevent development of new underground bulbs. This may be difficult with wet Oregon winters. If hoeing is not an option, 2,4-D applied before plants are 8 inches tall also provides effective control. Use caution when applying 2,4-D near nursery stock. Amine formulations of 2,4-D are safer (less volatile) then ester formulations. Make sure all herbicides are labeled for the site at which they are being applied.

Wild garlic bulbs can persist dormant in soil for 6 years. Nothing sprayed above ground can kill dormant bulbs beneath the soil surface. Complete control in a field infested with wild garlic cannot occur in a single year. Persistent management for at least 3 or 4 years (maybe as many as 6 years) is necessary to obtain complete control.

Summary

The overall concept in wild garlic control is simple: kill existing plants before they can produce the next generation and ultimately you will deplete the soil of this weed. Wild garlic control in practice is not quite so easy. If it were, I wouldn’t be writing about it and you wouldn’t be concerned about it. Wild garlic control is difficult and will require planning and persistence by the pest management supervisor at your nursery. Nonetheless, its biology and predictable timing of new bulb formation gives you an opportunity for control.

Disclaimer: This article is for educational purposes only. Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism. Always read the product label prior to using any herbicide.

Wild garlic in these potted trees will be an unwelcome suprise to the receiving nursery/landscape.
 
Throughout late fall and early spring, plants produce clumps of grass-like foliage. These clumps, growing in a hazelnut field, likely grow undisturbed. This is probably the reason they grow in such dense clumps (compared to the image to the right).
 
Wild garlic habit in a production nursery. Through frequent cultivations and disturbances, bulblets from these plants are spread throughout the field and emerge solitary (compared to the image to the left).
 
In early spring, flowering scapes emerge from foliage clumps. Scapes can contain either aerial bulblets, flowers, or both. Aerial bulblets and seed from flowers can reproduce new plants the following fall. Remove scapes before these reproductive parts are formed.
 
Some aerial bulblets form appendages before maturing. These may appear as germinated bulbs, but the green stem-like structures are merely extensions of the outer covering of the bulblets.
 
In the 3 images above: Top: wild garlic bulbs begin to swell in early to mid-March; Middle: with a little pressure, immature bulblets can be detached from the mother plant; Bottom: once detached, herbicides absorbed by the mother plant have no affect on the detached bulblet.
 
New bulblets are in the final stages of development at about the same time red maple (Acer rubrum) is flowering. Herbicide applications should occur prior to this, with a final application no later than when red maple begins blooming.
 
These pictures were taken September 31 at a local nursery. Foliage is emerging from bulblets and/or seed. These clumps will start producing more bulblets and seed in early spring, so control must occur before then.

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