Hemp Harvest Timing: Maximizing Yield and Quality in UK Conditions

Introduction

Harvesting industrial hemp at the optimal time is a critical decision for UK farmers, directly impacting crop quality, yield, and profitability. Unlike many traditional crops, the ideal harvest window for hemp is not a single point in time but varies significantly based on the intended end-use—be it for high-quality fibre, nutritious grain, or a dual-purpose system. This guide provides a comprehensive overview of determining the perfect harvest time for your fibre and grain hemp crops in the UK, covering key growth stages, end-use considerations, regulatory compliance, and logistical planning. For a complete overview of hemp farming, see our guide to hemp farming in the UK.

Understanding Hemp Growth Stages

Recognising the key growth stages of the hemp plant is fundamental to timing your harvest correctly. Each stage has distinct visual cues that signal the plant's development and readiness for different applications.

Harvesting for High-Quality Fibre

When growing hemp for fibre, the goal is to harvest when the plant's bast fibres have reached their peak quality in terms of length, strength, and fineness. This typically occurs during the flowering stage, well before seed maturation.

Harvesting too early results in finer but weaker fibres, while harvesting too late leads to coarser, less flexible fibres as lignin content increases. The optimal window is generally 70-90 days after seeding, as the plant is in early to mid-flower. At this point, the lower leaves may start to yellow. Farmers should also consider the soil they are using, as this can impact fibre quality. Read more in our guide to hemp soil requirements.

Harvesting for Maximum Grain Yield

For grain production, the focus shifts from fibre quality to seed maturity. The harvest should take place when the majority of the seeds are mature and the seed heads are well-developed. This is typically 100-120 days after seeding.

A key indicator is when the seed bracts (the small leaves enclosing the seeds) begin to turn brown and birds show increased interest in the crop. Shattering, where mature seeds fall from the plant, can be a significant cause of yield loss. Therefore, it's a balancing act between allowing seeds to mature fully and harvesting before substantial shattering occurs. The choice of hemp variety is also a critical factor in determining potential grain yield.

Dual-Purpose Harvesting

A dual-purpose harvest, aiming to capture both grain and fibre, presents a compromise. The harvest timing is typically dictated by grain maturity, meaning the fibre will be of lower quality (more lignified) than if harvested at its peak. However, this approach can maximise the overall economic return from a single crop. After combining for grain, the remaining stalks can be cut and left in the field for retting before being baled for fibre processing.

THC Compliance and Harvest Timing

In the UK, industrial hemp must not exceed a THC concentration of 0.2%. THC levels can fluctuate and tend to increase as the plant matures, particularly in the later stages of flowering and seed set. This is a critical consideration for farmers. [1]

Harvesting before THC levels spike is essential for compliance. Innovations in plant science, such as our Stackatrait technology and CRISPR gene editing, are being developed to create hemp varieties with ultra-low or zero THC, mitigating this risk for farmers. These advancements, supported by the Precision Breeding Act, will be crucial for the future of UK hemp farming.

Weather and Environmental Considerations

The weather plays a significant role in harvest timing. A dry period is ideal for harvesting, as it reduces the risk of mould and makes the operation of machinery more efficient. Farmers should monitor weather forecasts closely as the harvest window approaches and be prepared to act quickly when conditions are favourable. Unexpected rainfall can delay harvest and negatively impact both grain and fibre quality.

Equipment and Logistics

Standard agricultural machinery, such as combines and balers, can be adapted for hemp harvesting. For grain, combine harvesters are used, often with modifications to the header and settings to handle the tough, fibrous stalks. For fibre, specialised cutters or mowers are used. Planning the logistics of equipment availability and post-harvest transport is a key part of a successful harvest strategy. Consider participating in our farm trials to gain hands-on experience.

Post-Harvest Processing

After harvesting, grain must be dried to a specific moisture content to prevent spoilage and ensure long-term storage. Fibre stalks are typically left in the field for a process called retting, where environmental moisture helps to break down the bonds between the bast fibres and the woody core. Proper post-harvest handling is just as important as the harvest itself for ensuring a high-quality final product. [3]

Conclusion

Timing the harvest of industrial hemp is a multifaceted decision that balances crop maturity, end-use requirements, regulatory compliance, and logistical considerations. By understanding the plant's growth stages and having a clear goal for the crop, UK farmers can make informed decisions to maximise their returns. As technology and genetics advance, the process will become more streamlined, further enhancing the benefits of hemp farming and solidifying its place in a sustainable agricultural future.

References

1. GOV.UK. (2023). Hemp growing licence.
2. Home Office. (2022). Industrial hemp licensing policy.
3. AHDB. (2020). Industrial hemp: opportunities for UK arable farmers.