April 2006

Building the Chassis By Jim Faust

Final crop quality often depends onthe chassis built within the canopy.The chassis refers to the architecture,or scaffold, that develops as a resultof pinching. The scaffold is influencedby several factors including light interception,pinch technique and pinch timing. In thisarticle, I will review the basic principles of branchingand demonstrate how you can manipulatespring crops with pinch timing and technique.

Supply And Demand

A basic principle of branching is that thenumber of developing shoots is proportionalto the amount of resources availableto support those shoots. In other words, ifthe plant has a number of resources available,most axilliary buds below the pinchwill develop into lateral shoots. In contrast,if resources are limited, fewer shootswill develop. The most important resourceis light intercepted by the leaves; however,root growth and the supply of water andnutrients are also important.

Light

Light interception depends on theamount of light delivered tothe canopy and the spacing ofindividual plants. When plantsare grown under low light levelsthe number of branchesdecreases. Figure 1, above,shows the increase in branchingof vinca bedding plants asthe light level increases.

Spacing impacts branchingby altering both light interceptionand the light quality deliveredto axillary buds. Figure 2,right, demonstrates how spacingand plant size impact lightinterception. The actual datawere collected on New Guineaimpatiens (3- and 6-inches tall)grown in 4-inch pots andplaced pot-tight (high density) or at staggered(low density) spacing. Light interception was calculatedby making light intensity measurementsabove and below the canopy. Light interceptionobviously increases as plants get larger or spacingis tighter; however, it is very interesting toobserve how the light delivered to individualplants increases as spacing increases.

While this may be intuitive, it is noteworthy tosee that small plants placed at wider spacingdecrease the total light interception from 87 to 56percent, but the interception per plant increases 30percent (0.087 to 0.113 moles per day per plant).Similarly, large plants placed at wider spacingdecrease light interception from 97 to 70 percent,while the interception per plant increases 45 percent(0.097 to 0.140 moles per day per plant).

Additionally, a tight canopy will reduce theamount of red light that filters down to theaxillary buds. Buds that perceive filtered light(red less than far red light) do not break as wellas buds that intercept direct sunlight (red lightequals far red light). Thus, branching is inhibitedby tight canopies.

Spacing is a critical economical issue, so weare not suggesting that plants be grown at luxuriousspacing. Rather, we are trying merelyto demonstrate that crowded plants haveconsiderably lower light interception perplant and a low red-to-far-red light ratio;thus, it is to be expected that branchingwill also be greatly impacted. The takehomemessage is that the spacing at thetime of pinching will impact the plant scaffold.Overly crowded plants at the time ofpinch will certainly have lower shootcounts, so spacing needs to be consideredin regard to the time of pinching.

Container Volume

The impact of container volumeon branching is not oftenconsidered; however, plantsgrown in small containers maynot branch as well as the sameplants grown in larger containers.Figure 3, page 48, showsAfrican marigolds grown in fivedifferent containers. All plantswere grown at 6×6-inch spacing,and no pinch was performed.Container volume affects thewater and nutrition availablefor plant growth. The gaseousenvironment in the growingmedia is also likely to be different.Interestingly, the changes ingrowth occur before any significantroot restriction occurs, so being rootbound is not likely to be the causal factor.It may not be clear which factor is the mostimportant, but it is clear that container sizeimpacts branching, thus plants pinched prior totransplant may not branch as well as thosepinched after transplant due to the container volumeat the time of the pinch.

Pinch Height

While pinching appears to be arelatively simple concept, it reallyhas a big impact on the final plantquality in terms of branch number,plant height, flower number andtiming of flowering. To demonstratethis, we examined the effectof pinch height, pinch hardnessand pinch timing on nemesia.

Pinch height refers to the nodenumber remaining on the primarystem after the first pinch. Thenumber of nodes left on the stembelow the pinch obviouslyimpacts shoot number, but it isless obvious that this also affectsplant vigor and flowering. Todemonstrate this, we transplantedseveral species and then pinchedthem low, medium and high.These are relative terms. If a species producesmany nodes prior to setting a terminal flower,such as osteospermum, then a low pinch leftthree nodes, a medium pinch left six nodes and ahigh pinch left nine nodes below the pinch. If aspecies produces a lower node number prior toflowering, such as nemesia, then a low pinch leftone node, a medium pinch left three nodes and ahigh pinch left five nodes below the pinch. Thesame amount of tissue was removed during eachof the pinches. The pinches occurred on differentdays, since it took longer for more nodes todevelop prior to pinching.

Figure 4A, right, demonstrates the resultsobserved on nemesia. The low pinch (to oneremaining node) produced two shoots per plant,while the high pinch (to five remaining nodes)produced 10 shoots per plant. The node closestto the soil surface was always the most vegetativeand the slowest to flower. Vegetativenesswas quantified by recording the number ofnodes that formed on the shoot below the terminalflower, so the bottom node typically formedshoots that had seven or more nodes. The highestnode following the pinch was always themost reproductive, so these nodes producedshoots with 4-6 nodes.

In general, pinch height has little impact ontime to flower but has a great impact on thenumber of flowering stems per plant. For example,when two nemesia cuttings were grown ina 4-inch pot, the plants that were pinched toone, two, three, four or five nodeshad 3.7, 3.9, 5.9, 6.7 and 11.3 floweringshoots, respectively, per potafter 35 days.

Pinch Hardness

Pinch hardness refers to thenumber of nodes removed duringthe pinch, while the node numberremaining on the stem afterpinch remains the same (Figure4B, right). Thus, a soft pinchresulted in one node beingremoved from the shoot, while ahard pinch resulted in threenodes being removed from theshoot. Hard pinches are done onlater dates than soft pinches sinceadditional nodes must developon the stem prior to the pinch.

The time from pinch to firstflower was 18 days regardless ofthe pinch hardness; however, the hard pinchhad a later pinch date that resulted in extraproduction time. For example, the averageshoot for the soft pinch flowered in 36 days,while the hard pinch averaged 44 days toflower. The shoots that develop on the hardpinched plants also had an additional 1-2nodes per stem. Shoot number per plant wasnot affected by pinch hardness.

In general, pinch hardness does not affect totalshoot number, but does have a significant impacton the timing of flowering. For example, whentwo nemesia cuttings were grown in a 4-inch pot,the plants pinched to three nodes that had one,two or three nodes removed had 8.9, 8.5 or 4.8flowering shoots per pot after 35 days (all threepinch treatments had 9-10 total shoots per pot).Soft pinches minimize time to flower, while hardpinches increase time to flower. This occursbecause the uppermost node below a soft pinchproduces a shoot that has a lower node countprior to forming a terminal flower.

Pinch Hardness And Pinch Height

In the preceding two examples, the differentpinch treatments had to be performed on differentdates. This third example demonstrates the interactiveeffects of pinch height and hardness onpinches performed on the same date. For example,all plants had six nodes at the time of pinch andhad pinches that were hard and low to one nodeor soft and high to five nodes (Figure 4C, above).

The plants pinched toone node averaged 47 daysto flower, while the otherfour treatments averaged 36days to flower. The bottomnode was always the slowestto flower, and when theplants were pinched to justthis one node, floweringwas quite slow. The bottomnode produced shoots thataveraged 8-9.5 nodes perstem prior to flowering,while the nodes closest tothe pinch formed shootsthat averaged 4.2-6 nodesprior to flowering.

Pinch hardness and heightalso had a significant impacton the number of floweringshoots per plant. For example,when two nemesia cuttingswere grown in a 4-inchpot, the plants pinched toone, two, three, four or fivenodes had 0, 3.7, 3.2, 7.2 or11.3 flowering shoots per potafter 37 days.

In summary, branchingwithin the scaffold will beimproved if the light levelsare relatively high and thecanopy is open enough toallow light to penetrate intothe axillary buds. Pinchheight should be based onthe number of shoots neededto produce a satisfactorilyfull plant, and then a softpinch should be performed.This maximizes shoot productionand minimizes timeto flower. Pinching too lowresults in too few shoots(and may require a secondpinch) and those shoots may be more vegetativethan desired. Pinching too high can delay floweringby wasting time developing nodes on the primarystem rather than developing lateral shoots.

Typically, the worst-case scenario is to growa plant with two low pinches. By this we meanto pinch the primary stem to a low node numberand then follow up with another low pinchon the secondary stems. The resulting plantwill have a relatively slow growth rate due tothe small leaf area remaining after the pinch.Also, the shoot number will be low, and thoseshoots will be more vegetative, so floweringwill be delayed.