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Ecology of Hemp—what it needs and what it doesn’t.

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Ecology is defined as the branch of science that studies the interaction among organisms and their biophysical environment including both biotic and abiotic components. Much has been said about the wonder crop Hemp. From its origin and early use to the various processes it undergoes, we now have a fair amount of knowledge about the crop. But learning never ceases.

After getting the rudiments in place, we move to understand the ecology of hemp—its relation to its biophysical environment. This is important for those interested in growing and cultivating hemp as well as those who prefer to be informed consumers.

The aim of this article is to aware readers, irrespective of the class of supply chain they belong to, about the different geographical, topographical, climatic, and other environmental conditions that are best suited for an optimal hemp harvest.

Introduction—Getting the Basics Right

Hemp (also industrial hemp) is a strain of cannabis sativa plant species which is used to make an array of commercial and industrial products. Manufacturers cultivate hemp for the industrial uses of the products derived from it. Hemp is one of the fastest-growing plants with no or minimal need for pesticides.

Industrial hemp is unique in the sense that it is an annual crop. It, therefore, allows both orchid-type farms and extensive farming outfits to participate in the supply chain with some protocol and equipment adaptation.

Location & Topography

­­­For a profitable hemp harvest, ideal places are away from the equator and closer to the poles. It is best to avoid steep altitudes of more than 400m above sea level. Hemp requires flat fields with good perlocation in combination with hot days and cool nights.

Hemp can grow on as little as 1 hectare of land.


Industrial hemp is a versatile crop that can grow on a wide variety of soil types. The crop prefers a sufficiently deep, well-aerated soil with a pH value of 6 or greater. This is in addition to good moisture and nutrient-holding capacity.

Soils that offer a favourable water balance, good water permeability, and great nutrient-accumulation potential are very rich black mollisols, brown steppe, and brown rendiza soils.

The recommended phosphorous, sulphur, and calcium levels are medium to high (greater than 40 ppm), good (greater than 5,000 ppm), and not in excess (less than 6,000 ppm) respectively. It is possible that hemp may react poorly to the herbicides present in the soil. However, with time, it is possible to develop a reasonable yield of hemp on soils previously damaged. In this way, hemp helps in improving the soil health.

If hemp is grown in a soil with high Cadmium level, it should not be used for consumption. A well-irrigated area is ideal as it allows the plant to access water and prevents it from drowning at the same time.

In an ideal growing area, as the one described above, the roots will grow to a depth ranging from 15 to 30 cm.

How to prepare soil for growing industrial hemp?

Uniform germination of hemp seeds requires a fine and firm seedbed preparation. Ideal methods are drilling and conventional seedbed preparations. If the seeds are placed at a depth greater than 2 inches, the seedlings will not emerge uniformly. No-till systems often yield good results.

It serves well to employ tillage methods that retain precipitation, sustain porosity, incorporate the nutrients into the soil, and keep the surface smooth.


The most important nutrient for hemp is nitrogen. Hemp is a nitrophilic crop and requires an adequate supply of readily available nitrogen throughout the vegetative period. The nitrogen uptake is most demanding in the first 6 to 8 weeks. However, an extreme excess of nitrogen can reduce the fibre quality and quantity.

The next important nutrients for hemp are Potassium and Phosphorous. They are imperative for good elasticity and tensile strength of fibre cells and fibre quality respectively. The consumption of these nutrients is intensive in the process of flowering and seed formation.

Hemp requires less than 1/3rd of the nitrogen and less than ½ of the phosphoric acid required by cotton.

Climatic Conditions

Hemp requires a lot of moisture. The best climatic conditions for hemp are temperate weathers with distinctively hot summers and cold winters.


The temperature range for optimal hemp growth is between 19°C to 25°C. Hemp is relatively insensitive to cold temperatures and can withstand frost down to -5°C. The seeds of the plant can germinate down to 1°C to 3°C. For the earlier varieties to mature, hemp requires a high amount of heat.


Industrial hemp prefers a mild climate, humid atmosphere, and a rainfall of at least 25-30 inches per year. Since the amount of rainfall is unpredictable, it serves well to make use of early soil moisture.

Additional irrigation is required only if the plant is dry in the first few weeks. It is best to avoid flooding the young crop. Ensuring adequate irrigation often produces good hemp yields.

Seeds & Breeding

The seeds ideal for optimal hemp growth are fresh, bright, clean, and plump glossy. If the seeds are more than 2 years old, test for fertility.

Seed Density

The seed density determines the growth of industrial hemp. A high seed density gives the plant little space for branching out thereby resulting in thin, long stems. This condition is ideal for growing hemp for fibre. Depending on the soil type, soil fertility, and seed variety, ideal seeding rate is 250 to 400 viable seeds per square meter.

A low seed density allows the plant to branch on the sides with each branch bearing more seeds. This condition is ideal for growing hemp for seeds. Depending on the soil type, soil fertility, and seed variety, ideal seeding rate is 175 to 185 viable seeds per square meter.

Different varieties of hemp seeds produce different kinds of hemp. You can learn more about the seed varieties of hemp here.

Time of Seeding

It is the weather and climate conditions that dictate the best time to seed hemp. Taking plant rotation into consideration, hemp can be seeded two weeks prior to corn if the optimum soil conditions have been achieved. At the same time, it is important to ensure that seeding should not begin until soil temperatures have reached a minimum of 6°C to 8°C.

When growing hemp for fibre, it is best to seed as early as possible whereas when growing for grain, seeding should be done later to minimize stalk height.


In the Northern hemisphere, the best time to plant hemp is between March and May. In the Southern hemisphere, it is between September and November.

Sowing Method

The uniformity of the crop is assisted by regular spacing. The ideal seed depth is 2 to 5 cm; deeper for light soils. Cultivators use grain drills or other conventional seeding equipment to sow hemp seeds.

Seed Storage

Green or wet seeds need to dry soon after harvesting (preferably within 24 hours). The ideal moisture level to store them is between 9% and 10%. This helps in preserving viability.


There are three classifications of varieties:

  1. Monoecious varieties: when male and female flowers develop on the same plant.
  2. Dioecious varieties: when distinct male and female flowers grow.
  3. Female predominant varieties: come into existence on pollinating dioecious female plants with monoecious plants.

Generally, hemp plants are dioecious in nature.


Industrial hemp is usually classified into two varieties based on their use:

  1. Fibre cultivars: with long stalks and little branching.
  2. Seed/Grain cultivars: with shorter stalks, larger seed heads, and numerous branches.

You can learn about the planting conditions and characteristics of these, and other, cultivars of industrial hemp here.

Crop Rotation

Hemp is among the very few plants that can grow on the same land for years in succession. However, rotation with other crops always adds to the yield quality. Hemp is versatile in that it responds well to most preceding crops.


The best-recommended pre-crops for hemp are legumes, clovers, and lupins because they enhance the nitrogen level and organic matter content of the soil.

Companion Crops

Inter-planting hemp with assorted tree species provides wind protection and improves water balance.


Hemp can potentially benefit the crops grown after it. Therefore, it is generally grown before winter crops. The advantages here include weed suppression, loosening of soil, and positive effect on soil tilth.

Weed Control

When hemp is planted in well-drained, fertile soils under closely optimal temperature and moisture conditions, it germinates quickly. In such a condition it reaches a height of 30 cm in 3 to 4 weeks from planting thus providing 90% ground shade. This condition is best to suppress weed growth because of the exclusion of light from the soil.

Rapidly growing hemp (with final population of 200 to 250 plants per square meter can suppress nearly all weed growth including twitch grass.

However, weed suppression is not a permanent condition. It is possible that weeds may appear in the same field next year if alternate crops are planted.

When hemp is grown for grain, comparatively less weed suppression is achieved. The lower plant population and increased branches allow more light to penetrate into the soil thus aiding the germination of weed seeds.

Overall, industrial hemp is a low maintenance crop and requires no registered chemicals for weed control.

Diseases & Pests

Hemp plants can be vulnerable to various pathogens such as fungi, bacteria, nematodes, viruses, and other miscellaneous pathogens as well. These diseases often result in reduced fibre quality, stunted growth, and sometimes in plant death.

In fibre hemp crops, the most commonly occurring pests are Heliothis, Red-Shouldered leaf beetles, Lucerne Flea, and Green Veggie Bug. These infections are prevalent in clay soils and/or in regions where frequent watering occurs.

Nematodes, especially root-knot nematodes, can occur in the root systems of hemp in cropping soils. This infection can drastically reduce plant yield.

In the Southern hemisphere, the most common type of pathogen is fungi. Some of the well-known ones are Yellow leaf spot, hemp canker, grey mould, downy mildew, fusarium wilt, and fusarium stem canker.

The THC levels of hemp usually inhibit most viruses. Yet some mites that can attack hemp plant include spider mites and hemp russet mites. At times, birds can also act as a pest for hemp at early stages of germination, particularly to seed crops.


Hemp is usually ready for harvest in 3 to 4 months of time. Fibre hemp readies before seed hemp. If the ultimate use is for textile, it serves well to harvest the plant a little earlier. Otherwise, fibre varieties of hemp are harvested as soon as the last pollen is shed.

Harvesting for seeds occurs 4 to 6 weeks later than harvesting for fibre. The ideal condition is when 60% of the seed has ripened.

Smallholder plots can be harvested by hand. The harvesters cut the plant at 2 to 3 cm above soil and leave them on the ground to dry. Another common harvesting method, especially for larger plots, includes the use of simple and/or specially adapted cutter-binders.

The process of harvesting is often incomplete without the process of retting. The process of retting yields the best fibre bundles. Retting refers to the process of leaving the plant in the field to dry after harvest. Depending on the weather conditions, it can take 14 to 21 days to complete. The stem is turned over a couple of times during the process to allow for even retting. The process is complete when the fibres turn golden or greyish in colour and separate easily.


The output growth of industrial hemp relates directly to the levels of photosynthetically active radiation including heat. On well-drained loamy soils, the yield expectation ranges from 3 to 4 tons of baled hemp stalks per acre.

Work Skill

Though hemp is a low-maintenance crop, in orchid-style operations, as many as 2 people per acre may be required during harvest. This may increase during the process of cure drying.

On extensive farms, manual work skill requires is comparatively less.

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