With the planting of every crop there is a time of anticipation as we wait and watch for germination, seedling emergence, and the development of a good stand of plants to start the crop season. Successful seedling establishment is the first critical step for the production of any crop and it determines the foundation for either the success or failure of the future harvest. So, it is worthy to review some fundamentals associated with the process of seed germination and seedling establishment.
Most crops produced worldwide begin with the planting of a seed and management to sufficiently establish a satisfactory population of new plants in the field. Good seed germination and establishment is an essential and critical first step in the production of a good crop. Most crop seeds are desiccation tolerant while dormant and as a result can be stored and transported while preserving their ability to grow. Seeds carry the full genetic complement of the crop and serve as the genetic delivery mechanism for a crop production system. Farmers require seeds that will provide for a reliable and successful establishment of a crop.
An interesting aspect of seed germination and establishment is that seeds generally represent the condition of highest degree of resistance to extreme environmental conditions, while seedlings are most sucseptible. Viable seeds are dormant living organisms that contain living, embryonic tissue that is essential for germination. All fully developed seeds contain an embryo and a reserve supply of carbohydrates and nutrients to carryout germination and establishment and the full “package” is contained within a seed coat.
Seeds are stimulated to “wake up” and start the germination process when soil moisture and temperature conditions are appropriate for them to grow. The three cardinal points of vital activity for germination are: 1) a minimum temperature, below which no activity occurs, 2) an optimum temperature at which the highest germination occurs, and 3) a maximum temperature above which no germination takes place.
All seeds need correct moisture and temperature environments (Table 1) to initiate internal biochemical processes associated with the initation of germination. Soil-water content generally needs to be at least approximately 50–75% of field capacity. A fine-textured seedbed and good seed-to-soil contact are very imporant for optimal germination.
Good soil aeration that facilites good gas exchange between the germinating embryo and the soil environment is very important. Seeds respire just like any other living organism. Germinating seeds need oxygen and they produce carbon dioxide (CO2). The CO2 needs to be able to diffuse away from the seed and if the soil is compacted or saturated, aeration is inhibited and the CO2 respiration will not be able to dissipate to the atmosphere, and as a result the seeds can suffocate in the soil.
There are three major steps in the initial process of seed germination. These include: 1) imbition, 2) interim or lag phase, and 3) radicle and root emergence.
In the imbibition process seeds rapidly absorb water through the hilum of the seed and the seed coat will then swell and soften.
In the interim or lag phase the seeds internal biochemical and physiological system goes into action. Cells begin respire with this physiological activity and the seed begins to metaboloize its storehouse of energy and nutrient reserves to manufacture proteins other essential metabolites.
The radicle is the very first element of the rooting system that is developed and it is the first thing to emerge from the seed. In the field, we often refer to the radicle as the first “stinger” root that is developed. In the field, it is good to dig into the seedbed and examine early seedling development and look for healthy radicles, with clean, white, and turgid tissue.
In the process of radicle and root emergence, the cells of these tissues start to elongate and divide, the seed continues to imbibe water, and then it begins to extend out and away from the seed into the soil. These early root tissues, specifically the fresh root hairs become quickly capable of water and nutrient uptake. The young root tissues are very sensitive to the soil environment, particularly the soil solution. It is in this stage for example when seedlings are often very sensitive to salinity in the soil solution and disease infections.
When the radicle begins absorbing water from the soil solution, the seedling gains the capacity to extend a shoot from the seed towards the soil surface for emergence. In dicotyledonous (dicot) plants (generally referred to as broadleaf plants) the two seed leaves are extended above the soil for emergence. The section below the shoot and the cotyledons is referred to as the “hypocotyl” and the section of the shoot above the cotyledons is the “epicotyl”, Figure 1.
There are two basic methods of emergence for plants. In most dicot plants, a hook is formed in the section of shoot below the cotyledons which serves to pull the cotyledons and the growing point of the new plant up through the soil. Upon the reaching the soil surface, the hook will straighten and pull the cotyledons (seed leaves) and shoot tip into the air. For the farmer or agronomist in the field, that is a good sight to see. This method of emergence is called “epigeous or epigeal germination”. Examples of epigeal germination are cotton, sunflower, castor, and bean (common bean), Figure 1.
In other plants a process of “hypogeous or hypogeal germination” takes place. In hypogeous germination, the hypocotyl remains short and the cotyledons do not emerge from the seed and remain underground. Instead, the radicle and epicotyl axis will elongate and extend out of the seed coat. The epicotyl grows up through the soil and emerges above the soil surface and forms a epicotyl or “plumule” for the first leaf, Figure 2a and b.
All monocotyledons plants such as maize, rice, wheat, and coconut show hypogeal germination. It is also interesting to note that some dicot plants show hypogeal germination such as groundnut, gram, and peas.
In general, plants with epigeal germination, most dicot or broadleaf plants, are more sensitive to soil crusting and environmental inhibitions to emergence. So it is good to know what type of germination and emergence characteristics are natural for a given crop species and to track seedling development in the field accordingly.
As we begin any new crop season in the field and plant the seeds, it is good to review the basic process of seed germination and establishment. We can easily take all of this for granted until something goes wrong and we have problems in the field with emergence. After planting, it is good to monitor in-field seedling conditions and progression of growth and development, both above and below ground, and provide good management that supports the young plants as they get established and start their growing cycle.
Table 1. Soil temperature (oF) for crop germination. Adapted from Kemble and Musgrove (2006).
|
Crop |
Minimum oF |
Optimal Range oF |
Optimum oF |
Maximum oF |
|
Beet |
40 |
50–85 |
85 |
85 |
|
Cabbage |
40 |
45–95 |
85 |
100 |
|
Cauliflower |
40 |
45–85 |
80 |
100 |
|
Celery |
40 |
60–70 |
70 |
85 |
|
Chard |
40 |
50–85 |
85 |
95 |
|
Cucumber |
60 |
60–95 |
95 |
105 |
|
Eggplant |
60 |
75–90 |
85 |
95 |
|
Lettuce |
35 |
40–80 |
75 |
85 |
|
Melons |
60 |
75–95 |
90 |
100 |
|
Onion |
35 |
50–95 |
75 |
95 |
|
Parsley |
40 |
50–85 |
75 |
90 |
|
Pepper |
60 |
65–95 |
85 |
95 |
|
Pumpkin |
60 |
70–90 |
90 |
100 |
|
Spinach |
35 |
45–75 |
70 |
85 |
|
Squash |
60 |
70–95 |
95 |
100 |
|
Tomato |
50 |
70–95 |
85 |
95 |
Figure 1. Seed germination and plant establishment for a plant with epigeal
germination and emergence, which is common with most dicot or broadleaf
plants.
Figure 2a. Examples and comparison of epigeal and hypogeal germination.
Figure 2b. Examples and comparison of epigeal and hypogeal germination.
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Spinach ‘Meerkat’ was seeded, then sprinkler-irrigated to germinate seed Jan 13, 2025 on beds with 84 in. between bed centers and containing 30 lines of seed per bed. All irrigation water was supplied by sprinkler irrigation. Treatments were replicated four times in a randomized complete block design. Replicate plots consisted of 15 ft lengths of bed separated by 3 ft lengths of nontreated bed. Treatments were applied with a CO2 backpack sprayer that delivered 50 gal/acre at 40 psi to flat-fan nozzles.
Downy mildew (caused by Peronospora farinosa f. sp. spinaciae)was first observed in plots on Mar 5 and final reading was taken on March 6 and March 7, 2025. Spray date for each treatments are listed in excel file with the results.
Disease severity was recorded by determining the percentage of infected leaves present within three 1-ft2areas within each of the four replicate plots per treatment. The number of spinach leaves in a 1-ft2area of bed was approximately 144. The percentage were then changed to 1-10scale, with 1 being 10% infection and 10 being 100% infection.
The data (found in the accompanying Excel file) illustrate the degree of disease reduction obtained by applications of the various tested fungicides. Products that provided most effective control against the disease include Orondis ultra, Zampro, Stargus, Cevya, Eject .Please see table for other treatments with significant disease suppression/control. No phytotoxicity was observed in any of the treatments in this trial.
At the UC Cooperative Extension Automated Technology Field Day in Salinas, CA a couple of weeks ago, several automated technologies were showcased operating in the field for the first time to a general audience. One of the “new” machines designed specifically for in-row weeding in vegetable crops is highlighted here. I’ll discuss other technologies in future articles.
The first is an autonomous robot that uses lasers to kill weeds (Fig. 1a). The machine, developed by Carbon Robotics1, identifies weeds from camera captured images of the bedtop using artificial intelligence techniques. The device is equipped with eight CO2 lasers that emit laser beams roughly 0.25” in diameter in short, < 50 millisecond bursts. As such, it is best suited for controlling small weeds (< 3-4 leaf stage) in high density crops such as baby leaf spinach, spring mix, carrots, and onion. The machine worked well in the demonstration plot, annihilating small weeds, turning plant material into black charcoal (carbon!) and grey ash, and causing significant damage to and potentially killing larger weeds (Fig. 1b).
The percentage of weeds targeted and travel speed (significantly less than 1 mph) of the 80” wide (1 wide bed) machine was comparable to that shown in videos on the company’s website (https://carbonrobotics.com/). The version the company is marketing, however, is a 20’ wide (3 wide beds) tractor pulled implement, and according to sales representatives, has upgrades that increase travel speed. This combination should provide adequate machine productivity (acres/hour).
Company reps reported that the first production units are being delivered to commercial farms presently. Some will be operating in the Yuma area this fall. If interested, I would be more than happy to work with you to help conduct and design experiments for assessing the machine’s performance – % weed control by species, % of weeds targeted, hand weeding labor savings, machine work rate (acres/hour), etc. Please feel free to contact me anytime at siemens@cals.arizona.edu.
____________________
[1] Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.
Fig. 1. Autonomous laser weeding machine developed by Carbon Robotics1 a) demonstration at UCCE Automated Technologies Field Day and b) image of bedtop weed control post treatment.
London Rocket (Sisymbrium irio)
If you drive through the Valley of Yuma Arizona this week you will see a significant amount of London rocket, a winter annual broadleaf weed from the family of the Brassicaceae. We can see it in already harvested lettuce fields, edges of alfalfa fields, on roadsides, orchards, and gardens.
Why London Rocket? According to some sources the common name "London rocket" allegedly comes from its abundance after the Great Fire of London in 1666 even though years later Dr E J Salisbury failed to find specimens in a study in 1940 [1].
The seed leaves (cotyledons) are oval, around 2–6 mm long, without pubescence. True leaves are a bit larger than the cotyledon, has smooth edges or a few weak teeth, and is on a long stalk. Leaves are alternate on the stem.
The mature plant grows approximately 24-36” tall in our area. It grows erect and ranches are mostly close to the base of the plant. The leaves have very little pubescence and have opposite lobes. Right now, we can see its abundant yellow flowers in our area and grow in clusters at the top of the stems (pic below). The fruits are 1-1.5” seedpods that are straight or a little curved and about 1mm wide.
Here in the “Lettuce capital of the world” this weed can be a problem because of it is highly competitive.
In lettuce production the application of Kerb (Pronamide) has shown good London rocket control when applied properly. This means not too early or with too much sprinkler irrigation to avoid leaching. Delayed application is often needed, and the location of the herbicide is critical for best performance. Approximately 1-3 days after starting sprinklers for early plantings (Aug-Sep) and from 3-6 days for mid-season plantings (Oct-Nov) [3].
Interestingly, medical researchers tested some extracts of seeds and found it showed significant antipyretic and analgesic effects and antibacterial action [2]. Other medicinal properties have been reported such as the reduction of swelling and use for cleaning of wounds, rheumatism relief, chest congestion, cough, detoxify the liver and spleen [1].
When weeds like London Rocket are infesting our lettuce or alfalfa fields, they can be such a nuisance. But it's possible that in other parts of the world people are looking for them as beneficial herbs.
London rocket seedling and mature plant in alfalfa field.
References:
1. Retrieved from: https://en.wikipedia.org/wiki/Sisymbrium_irio
2. Antipyretic, Analgesic and Antimicrobial Studies on Sisymbrium irioS. B. Vohora, S. A. H. Naqvi, I. Kumar (1980) Institute of History of Medicine & Medical Research, Tughlaqabad, New Delhi (India).
3. Retrieved from: https://www.farmprogress.com/timing-critical-kerb-herbicide
