A primary bud with one medium-sized inflorescence that will grow into a bunch.

Some Australian studies suggest that the critical factor is the number of days withmaximumtemperatures between82° and 90°F during key time periods, corresponding roughly to the second through fourth weeks of May for Thompson Seedless in the San Joaquin Valley.4

Cold spring temperatures are probably the factor most commonly causing areawide lowfruitfulness the following year.

Light and shading — More buds in a shoot produce flower clusters when more light falls on leaves of that shoot. Light on the leaves is more important than previously thought than light on the buds. This seems to bemore a function of total accumulated light rather than a peak of maximum intensity. In other words, a shorter day-length or several hours or days of cloudiness could reduce fruitfulness. The Australian study noted above rated solar radiation along with days of favorable temperature as the two critical climatic factors.4 Shade produced by heavy foliage also reduces bud fruitfulness, but this is not occurring in the first few weeks of growth.1

Temperatures are typically cool during cloudy periods, and the two factors in combination can be expected to reduce fruitfulness more than either alone.

Carbohydrate reserves — Stored carbohydrates are thought to have a strong influence on the differentiation of inflorescences in young buds.2 Carbohydrates are stored in the roots, trunk, cordons, and canes or spurs. These carbohydrates were stored the year before bud differentiation — two years before the crop. This means that growing conditions and vine strength two years ago have a strong influence on this year’s fruitfulness.

Buds seem to be a weak carbohydrate sink compared to the growing shoot.2 Rapid shoot growth can be expected to draw carbohydrates away from the buds, reducing their fruitfulness.

Sometimes two complete compound buds develop at the same node, approximately equal in size and parallel with each other and the direction of shoot growth. This has been noted most commonly in Pinot Noir, but also in Cabernet Sauvignon, Syrah, Merlot, and occasionally Chardonnay.2

The compound buds develop sequentially as the cane grows. Each compound bud develops over a twoto three-month period that starts soon after its node first appears at the tip of the growing shoot. The bud then stops growing and remains dormant until bud break the following spring.

Most of the branching of the rachis occurs in the flower cluster primordia before the buds go dormant in summer, but the flower initials, which become the actual flowers along the rachis, form after winter around bud break. This means that conditions at bud break influence the final number of flowers formed. Otherwise the size of the flower cluster at bud break is determined the prior year.1,3,4

Each flower cluster primordium develops from a tiny lump of tissue called an “anlage.” Each anlage develops into either an inflorescence (flower cluster) or a tendril. The inflorescence can be distinguished microscopically when

the bud is about 10 nodes from the tip of the growing shoot, but the vine’s “decision” to form an inflorescence comes much earlier. For example, the correlation of temperature with inflorescence formation is strongest in the third visible node from the apex, which has a leaf of only about 1.5 cm. in diameter.1,3,4

In other words, bud fruitfulness in the first few buds is determined by vine physiology and the environment of the shoot in the first weeks following bud break. All of the two to five buds that will constitute next year’s spurs differentiate before bloom. In spur-pruned vines, this is next year’s entire crop. For cane-pruned varieties, next year’s crop will include buds that differentiate further out on the cane, so they pass through the critical environmental conditions at a later time. In Thompson Seedless, differentiation in the first 15 nodes is determined by around the middle of June.4

Several factors influence inflorescence differentiation. Some of these factors are well-documented; others are merely suggested by the evidence.

Temperature — Both cold and hot temperatures inhibit inflorescence, but in the weeks following bud break, cold temperatures are a threat.