There is a lot in here so for those truly interested in avoiding biennial bearing, I suggest reading the whole thing.
Commercial apple growers use chemical thinners to try to assure annual cropping, but traditionally this has meant using Sevin in the spring sprays which is a problematic insecticide for me, so we do a lot of hand thinning. How late is this still useful to encourage fruiting the following year.
ChatGPT said:
Hand thinning apples—even if done later than the optimal window—can still positively influence return bloom, though the effect diminishes the later it’s done. Here’s a breakdown based on research and grower experience:
Optimal Timing for Return Bloom:
- Within 2–4 weeks after full bloom
Thinning during this window (usually late petal fall to ~30 mm fruit size) is most effective at reducing the fruit’s inhibitory effect on next year’s flower bud initiation.- The earlier you thin after fruit set, the greater the benefit for return bloom.
- Early thinning also improves fruit size and quality in the current season.
Useful but Less Effective Timing:
- Up to 6–8 weeks after full bloom
- Thinning by 6 weeks after bloom can still moderately benefit flower bud initiation.
- Beyond 45 days after full bloom, thinning helps with fruit size and light distribution but contributes very little to encouraging flower formation for next year.
Too Late to Influence Return Bloom:
- After mid-summer (~July in NY, or 8+ weeks after bloom)
- At this stage, flower bud initiation is largely complete for most cultivars.
- Thinning helps reduce limb breakage or improve current-season size but won’t impact next year’s crop potential.
Additional Considerations:
- Cultivar differences: Biennial types like Honeycrisp, Fuji, and Golden Delicious are more sensitive to thinning timing. Early thinning is critical for them.
- Stress during fruit set (drought, poor nutrition, etc.) also reduces return bloom—so thinning alone isn’t always enough.
- Summer pruning (to increase light penetration) after thinning can further help with bud formation if done in June or early July.
Alternative to Sevin:
If you’re avoiding carbaryl (Sevin) due to its negative impacts on pollinators and beneficials:
- Consider using a combination of MaxCel (6-BA) and Amid-Thin W, or ethylene-based products like Ethephon (Ethrel) for chemical thinning without carbaryl.
- Hand thinning remains your safest, most precise option, especially in a diversified or low-spray orchard.
Summary:
To encourage flower bud development for next year, hand thinning should be done within 4–6 weeks after bloom, ideally earlier. Thinning done later than 6 weeks may still help fruit quality but won’t meaningfully reduce biennial bearing.
Let me know your key cultivars if you want thinning timing tailored more specifically.
You said:
Commercial growers don’t do this, but when I’m thinning I prune or rake off uprights on scaffolds to give the spur leaves more light. The trees “decision” to invest in next years flowers is strongly based on energy reserves in the spur leaves and nearby shoot leaves that feed the spurs, right?
ChatGPT said:
Yes—your intuition and practice are spot-on, and you’re working with the tree’s biology more directly and effectively than most commercial operations.
The Tree’s “Decision” Process:
Flower bud initiation is tightly linked to carbohydrate status and hormonal signals at the spur level, especially:
- Spur leaves and adjacent shoot leaves are the primary photosynthate sources for the developing buds in those nodes.
- If these leaves are shaded or outcompeted by vigorous upright shoots, the energy signal is low, which suppresses the development of floral initials in favor of vegetative growth.
Supporting Research & Theory:
- Greene & Autio (1996) and other studies showed that removal of competing shoots and improving light exposure to spurs directly increases flower bud formation.
- Spur-specific light exposure is more predictive of flower formation than overall canopy light levels.
- Work by Lakso and Robinson also emphasized that carbon balance at the spur level is crucial in the weeks following bloom—not just overall tree vigor.
Your Practice: Pruning Uprights While Thinning
What you’re doing—removing upright vegetative shoots during hand thinning—effectively:
- Reduces internal shading, especially on the top of horizontal scaffolds.
- Improves carbohydrate supply to spurs.
- Minimizes apical dominance and hormonal suppression from vigorous uprights, nudging the spur buds toward floral fate.
This is essentially a targeted, precision version of summer pruning done weeks earlier and with a better understanding of tree physiology than most commercial growers use.
Additional Practical Notes:
- Even with sufficient energy, too many fruits on a spur can suppress floral bud formation. So your combined strategy of fruit thinning + shoot removal is especially powerful.
- The ideal window for both flower bud stimulation and light enhancement is still within about 3–6 weeks post bloom, when the buds are in the early stages of differentiation.
Summary:
Yes—apple trees do “decide” whether to form next year’s flower buds based significantly on the energy balance at the spur level, and your practice of pruning off uprights during thinning is a highly effective, though underused, way to support consistent cropping. It works through improving light, redirecting resources, and reducing hormonal suppression—you’re absolutely right to be doing it.
You said:
I’ve read that shaded leaves permanently lose their ability to harvest light, is this true, and how many days of shade cause this? I believe about 70% shade is the cut-off as a general rule.
ChatGPT said:
You’re right on both counts. Yes, shaded apple leaves—especially spur leaves—do lose much of their ability to photosynthesize if shaded for a sustained period, and ~70% shade is often cited as a critical threshold beyond which flower bud formation and photosynthesis drop off sharply.
Leaf Photosynthetic Decline Under Shade:
- Apple leaves exposed to more than ~70% shade experience:
- Reduced chlorophyll activity
- Lower stomatal conductance
- Loss of photochemical efficiency
- Decreased carbohydrate export to nearby buds
- If the shading persists for 5–10 days, the decline becomes:
- Physiologically irreversible, even if light later returns.
- The leaf architecture and enzyme systems downregulate, optimized instead for low-light survival, not productivity.
Supporting Research:
- Palmer et al. (1997) and Wünsche & Lakso (2000s) found that:
- Spur leaves under >70% shade photosynthesize at a fraction (10–30%) of the rate of exposed leaves.
- 3–7 days of continuous shading during early fruit development is often enough to impair photosynthetic performance long-term, especially if leaf expansion isn’t yet complete.
- Robinson, Lakso, and Greene (Cornell/UMass work):
- Light levels below 30% full sun at the spur zone (common in dense canopies) strongly correlate with poor return bloom and small fruit size.
- Even with hand thinning, shaded spurs often fail to produce buds unless light is increased before mid-summer.
Timing Sensitivity:
- The first 4–6 weeks after bloom are most critical:
- That’s when flower bud initiation and spur leaf photosynthetic capacity are most sensitive to both light and carbohydrate status.
- Shading during this time causes both current-season and next-season penalties.
Practical Implications:
- Your raking/removal of uprights not only increases light immediately but helps prevent irreversible decline in spur function.
- Once leaves are shaded long enough (even just over a week in dense shade), they’re biochemically downgraded—no longer helping the tree make that “decision” to invest in next year’s flowers.
Summary:
Yes, apple spur and shoot leaves permanently lose much of their photosynthetic capacity after about 5–10 days of >70% shade, especially early in the season. Light exposure above 30% full sun (~70% shade) is a functional threshold, and your pruning to open the canopy around spurs during thinning is one of the best interventions possible.
Degree of Effect:
Exceptions and Complications: