Seedling Vigour & Rate the Seed Lot

The seedling vigour index is a single number that rates how strongly a seed lot establishes. It multiplies germination percentage by average seedling length, so it rewards seed that both germinates well and grows away vigorously. A higher index means faster, more uniform stands; a lower one warns of weak, patchy establishment.

The standard vigour index (sometimes called vigour index I) is germination percentage × seedling length. For example a lot with 90% germination and 12 cm seedlings scores 90 × 12 = 1080. Some labs also report vigour index II using seedling dry weight instead of length. This tool uses the length-based index, the most common field measure.

It depends on the crop and the units used, so vigour indices are most useful for comparing lots of the same species tested the same way. Within a crop, the lot with the higher index will generally establish faster and more evenly. Track your own lots over time to learn what 'good' looks like for your seed and conditions.

A high germination percentage alone doesn't guarantee a strong crop — seeds can sprout yet produce weak, slow seedlings that struggle against soil crusting, cold or competition. Vigorous seedlings emerge faster, more uniformly and tolerate stress better, giving better stands, easier weed control and higher yield potential. Vigour is the practical test of true seed quality.

Run a standard germination test: place a counted sample of seeds on moist paper or media at the recommended temperature, and after the set period count normal seedlings to get germination percentage and measure their length (often root plus shoot). Use the averages in the calculator. Test a representative, well-mixed sample for a meaningful result.

Germination percentage measures how many seeds sprout under ideal lab conditions. Vigour measures how well they perform — speed, uniformity and stress tolerance — which predicts field establishment far better. Two lots with the same germination can differ greatly in vigour, which is exactly what the vigour index captures by combining germination with seedling growth.

Vigour indices are best compared within the same crop and the same test method, because seedling lengths and typical germination differ by species. Comparing an index for maize against one for lettuce isn't meaningful. Use it to rank your own lots of a given crop and to track how storage and age affect them.

Vigour declines before germination does as seed ages. An old lot may still germinate acceptably in the lab yet have lost the vigour to establish well in cold or crusted soil. Re-testing the vigour index of stored seed each season catches this decline early — pair it with the Seed Viability Decay tool to project shelf life.

High-vigour seed gives the best possible start, but it can't overcome a poor seedbed, wrong depth, drought or disease. Use a strong vigour index alongside good seedbed preparation, correct sowing depth and rate, and timely moisture. Vigour stacks the odds in your favour; agronomy still does the rest.

They're solid, repeatable indicators when you test carefully. Vigour indices vary with sample, test temperature, media and how length is measured, so standardise your method and use the index for comparison rather than as an absolute. Test several samples per lot and re-test over time for the clearest picture.