Germination is the renewal of enzymatic activity that results in cell division and elongation and, ultimately, embryo emergence through the seed coat. Germination is triggered by absorption of water through the seed coat. Corn kernels must absorb (imbibe) about 30 % of their weight in water before germination begins.
Less than optimum absorption of water (perhaps due to a rapidly drying seed zone) may slow or stop germination. Repeated wetting and drying cycles can decrease seed viability.
By comparison, soybeans must imbibe about 50 % of their weight in water. But since soybeans are approximately 2/3 the weight of corn kernels, the total amount of absorbed water required for germination is relatively similar.
The visual indicators of germination occur in a distinct sequence and are strongly influenced by soil temperature. Germination and emergence will occur in fewer calendar days when the seed is planted into warm soils than in cold soils. However, the thermal time from planting to emergence is fairly predictable, especially if estimated using soil temperature.
Measured in terms of Modified Growing Degree Days (GDD), corn typically emerges in 115-120 GDD (equivalent to about 65 GDD Celsius) after planting.
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In terms of calendar days, the fewest number of days after planting I have ever seen corn emerge is about 4 and that was with average daily soil temperatures that averaged 75F throughout the ENTIRE [planting to emergence] period.
The slowest I have seen corn emerge is about 35 days after planting and that was with average daily soil temperatures averaging 50F throughout the ENTIRE [planting to emergence] period. The latter observation should give you pause for thought considering that “50F” has always been considered lower temperature threshold for planting corn.
Yes, corn will germinate at 50F, but if soil temperatures fluctuate after planting above and below 50F, emergence can take a very long time.
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Sequence of Visual Indicators
- The radicle root emerges first, near the tip end of the kernel, within two to three days in warm soils with adequate soil moisture or much longer if soil temperatures hover at or below 50F (10C). In cooler or drier soils, the radicle root may not emerge until one to two weeks after planting.
- The coleoptile (commonly called the “spike”) emerges next from the embryo side of the kernel within one to many days of the appearance of the radicle, depending on soil temperature. The coleoptile is a rigid piece of plant tissue that completely encloses the four to five embryonic leaves (plumule) that formed during grain development of the seed production year. The coleoptile initially negotiates its way toward the dent end of the kernel by virtue of the elongation of the mesocotyl. The plumule leaves slowly enlarge and eventually cause the coleoptile to split open as it nears the soil surface.
- The lateral seminal roots emerge next and initially elongate towards the dent end of the kernel. Even though these roots and the radicle root are technically nodal roots, they are considered part of the seminal (seed) root system and not part of the permanent nodal root system that develops later. The first so-called “permanent” roots begin elongating from the first node at the crown of the seedling at approximately the V1 leaf stage (1 leaf with visible leaf collar) and are clearly visible by V2.
Understanding the natural sequence of the visual indicators of germination can help you troubleshoot problems with germination or emergence.
When temperatures are optimum, the visual indicators of germination may appear on nearly the same day. Excessively cool soils may delay the appearance of the coleoptile and lateral seminal roots for more than a week after the radicle root emerges.
It is not uncommon in cold planting seasons to dig up kernels two weeks after planting and find only short radicle roots and no visible coleoptiles.
When excessively cold and/or wet soils delay germination and/or emergence, the kernel and young seedling are subjected to lengthier exposure to damaging factors such as soil-borne seed diseases, insect feeding and injury from pre-plant or pre-emergent herbicides and carryover herbicides from a previous crop.