Hemp responds well to soil moisture availability during the growing season. Distribution of precipitation over the growing period plays a pivotal role in achieving high yields.
Water required by a Hemp crop varies from spring to fall, year to year, and location to location because of the influences of humidity, temperature, wind, light and soil type. Crop evapotranspiration is primarily influenced by the stage of growth and amount of ground cover.
Limited research from Europe shows hemp requires around 500–700 mm of precipitation per growing season (Bosca and Karua, 1998) which is met entirely by rainfall. Research has shown that on average 300–500 litres of water are required for the production of 1 kg of dry matter. By contrast, cereal grains require 350 to 650 and corn 500 to 800 mm of precipitation per growing season.
Hemp yields will be highest when there is adequate soil moisture throughout the growing season. Adequate soil moisture is defined as maintaining 50% or more of the available soil moisture in the root zone.
Excessive rains after seeding, particularly on heavy soils, may have detrimental effects on crop establishment as hemp does not tolerate “wet feet” during early stages of development; therefore, hemp should be grown on well-drained soil to avoid the possibility of waterlogging or even full saturation of soil. When a field is water saturated, the crop stalls, which will affect its ability to compete with weeds. Hemp plants turn yellow and cease development, often resulting in drastic yield penalty or even in total crop failure.
Stunted, necrotic plants 22 days after seeding
Stunted, prolonged wet weather conditions on poorly drained soil led to total crop failure (59 days after seeding)
Stunted, prolonged wet weather conditions on poorly drained soil led to total crop failure (59 days after seeding
The growing point at the top of the plant is the first to be affected.
After an extended wet period, some plants may resume growth, but the plants will have reduced vigor, nutrient loss, chlorosis and plant development will be poor throughout the growing season. Weed competition will become a problem, plant stands will be less and yield will be significantly affected. If the wet period is short and there is good drainage, plants should recover with minimal to no effect on plant development and yield ability. As the plants become older (over about 25 cm (12 inches tall)), they can tolerate heavy wet conditions a lot better.
Hail can affect any crop during the growing season and the extent of the damage will vary depending on the growth stage of the crop.
Hail damage and recovery
In hemp, the frequently experienced storms from late June to mid-July can cause mild to severe damage depending on storm intensity and hemp usage type. During this period, short stature, grain type varieties (i.e. Finola) are more resilient to hail storms than tall, fibre type varieties that rapidly elongate (up to 10-15 cm per day (4 - 6 inches)) and their stem structure is more prone to physical damage by the impact of hail stones or gusty winds.
(Photo x Rapidly growing fibre type hemp cv. Crag. Picture of the same spot were taken in mid-July five days apart. )
Intense storms can break off the top portion of the plant and destroy growing points of the leader, break/bruise the stems and puncture or shatter the leaves resulting in severe losses of fibre and grain yield and quality. If hail affects the actively growing crop early in the season, plants may have enough time to recover and reduce the extent of a potential crop failure which is more commonly seen with August hail storms that impact more mature plants.
(PHOTOs (2) (place the two pics side by side) Canopy damage of fibre type variety one day early July storm)
Despite the damage of apical meristems (plant tops), moderately damaged plants can continue to grow by sending out two shoots from the node.
Damage and recovery of fibre type variety after early July storm. Photo taken 19 days after the event. Regrowth of moderately damaged plants (left), irreversibly damaged stems (right)
The damaged main stem will die and seal off the top of the stem. These shoots, if they have time, will develop new leaders that may set seed heads. In this instance, time to grain maturity will be extended, while fibre quality and often yield from the secondary branches will be reduced.
Regrowth of fibre type variety after early July storm. As a result of a damaged apex (brown tissue on the left panel) secondary branches were grown from two upper nodes generating a shorter and bushier plant (top on the right panel; intact plant on the bottom). Photo taken 29 days after the event.
Severely damaged, broken plants may not recover, hence the overall yields on hailed fields will be reduced.
Milder storms have typically less impact on hemp. The head will “goose neck” and grow back towards the top of the crop canopy. This will delay maturity and yield, but will often make a reasonable recovery. The goose neck heads do pose a problem at harvest since they are lower in the crop canopy. The combine operation can only take in a certain length of stalk, so often many of these recovered heads will have to be left in the field.
Mildly hail damaged hemp can recover causing only minor yield penalty. (Goose Necking)
An early spring seeded hemp crop is more tolerant to frost than a later seeded hemp crop. Hemp germination can start as soon as temperature exceeds the zero mark (1 to 2°). A Russian study reported that germinated seeds can withstand -5° frosts for a period of two weeks and up to -15° for 24 hours under low soil moisture conditions. Plants at the seedling stage easily tolerate frosts to about -7°. Numerous volunteer plants often found in fields in the subsequent year are the best indication of fairly high frost hardiness of hemp at early developmental stages.
Early fall frost that frequently affects many areas of the Canadian Prairies could be beneficial for hemp growers. Below zero temperatures act as natural desiccant, facilitating maturation of the crop and permitting earlier and more effective combining. It is particularly advantageous in the case of late maturing, fibre type varieties where leaves and bracts can remain green and the seeds are mature. A killing frost causes leaf drop and desiccation of the seed head, leaving bare stems and reduced biomass volume to be combined.
Leafless stems of X59 as a result of a killing mid-September frost.
Hemp fields affected by frost will dry down rapidly. Dependant on weather, the moisture content can drop 4% to 8% in 4 or 5 days. If harvest is delayed, the crop can become too dry and difficult to combine because the fibre will separate causing the fine fibres to wrap on the harvest equipment. Experienced producers will be prepared to start combining hemp about 4 days after a frost depending on the severity of the frost.