In the London area alone, we remove an impressive amount of up to 1,700 tonnes of aquatic weed from the canal network every year.
To put it in perspective, that's equivalent to the weight of 135 double-decker buses. We tackle this challenge primarily by utilising specialised craft designed for mechanical weed removal. The typical season for this important task runs from April to September, with weather conditions playing a significant role. Unsurprisingly, when sunshine and high temperatures prevail, the weed tends to grow rapidly. However, we also engage in some weed removal during the winter months, specifically targeting the invasive Floating Pennywort.
Aquatic weed poses a serious threat as it can quickly clog up the canal network, making it difficult for liveaboard boats to navigate. Additionally, our business boating customers and leisure and recreational users of the canals are adversely affected by excessive weed growth. When the waterways are covered with a thick layer of weed, it blocks out sunlight, reduces ecological diversity, and deprives the water of oxygen, harming aquatic life. Some of the weed species are non-native and highly invasive, so it is our obligation to remove them and prevent their spread to adjacent waterways.
Types of Aquatic Weed
Introduced to the UK in the 1980s by the aquatic nursery trade, this fleshy-stemmed plant forms dense floating mats of lush foliage. It is an extremely invasive plant causing significant issues in our waterways nationwide, obstructing canals, rivers, and other water bodies. Similar to Japanese knotweed, Floating Pennywort can grow from minuscule fragments, making its removal a challenging and costly endeavor.
Duckweed is a familiar sight to most pond owners. These tiny, rounded leaves, usually around 0.5cm across, rapidly multiply to cover the surface of any still or slow-moving body of water. Some species even overwinter at the canal's bottom, resurfacing as soon as spring arrives. The weed mass can double in size within a few days. Our operatives are mindful of the need to prevent the distribution of weed across the network or onto adjacent water bodies. Duckweed is particularly prone to being blown around by the wind, so it's not uncommon for us to chase it up and down the canal, diligently removing it and controlling its spread.
Also known as fairy fern, this popular pond plant originally hails from North and Central America. However, there's nothing delicate about this floating nuisance. Water Fern forms dense mats on the water's surface, often deceivingly appearing as solid ground. It can thrive in any depth of still or slow-moving water and has the ability to rapidly invade new regions. Within days, Water Fern can double in area, dominating many waterways.
Blanket Weed is actually a type of algae that forms clumps of fine, hair-like strands, scientifically known as filamentous algae. It thrives in nutrient-rich water, deriving its sustenance from various natural and less natural sources, such as organic debris or fertiliser runoff. Blanket Weed typically floats on or near the water's surface and tends to cling to other vegetation. When it gets entangled in a boat's propeller, it forms strong, rope-like structures that can quickly bring the boat to a halt.
We must also consider the role of watersports equipment in facilitating the spread of invasive weed across the network. Implementing biosecurity measures like "check, clean, dry" is crucial to prevent this from happening.
The quantity of aquatic weed in the canal closely correlates with temperature and sunshine hours. Typically, there is a delay of about a week or two between high temperatures and sunshine hours leading to a surge in the tonnage of weed being removed from the canal.
Where Do We Work?
The London area is divided into three smaller operational areas: London East, comprising the Lea Navigation and Limehouse Cut; London Central, encompassing the Regents Canal and Paddington Arm; and London West, covering the Grand Union Canal.
To combat aquatic weed, we employ two types of craft for mechanical removal on the canal network.
A weed harvester is particularly effective when dealing with relatively thick layers of duckweed. However, if the weed coverage is very light, it may get pushed away from the boat as it moves forward. The harvester utilises a constantly rotating belt to collect the weed from the water's surface and convey it into a storage hold. It can store up to 3.5m³ or approximately 3 tonnes of weed before offloading it via the belt into a hopper (a large floating skip) or onto the bank. Harvester craft are equipped with cutting bars mounted to the front of the conveyor, enabling them to cut rooted weed and break up larger weed mats.
Conver boats are highly maneuverable craft and prove to be very effective in tackling various types of aquatic weed. They can quickly adapt to different attachments based on the nature of the work, such as cutting heads or baskets.
Our goal is to prioritise placing weed on the bank whenever possible and composting the weed we remove from our network due to space constraints or other reasons. Historically, a significant portion of the weed, the majority, was sent to landfill, which proved to be an inefficient, expensive, and environmentally unsustainable process. It involved transferring the weed to a hopper, transporting it to a wharf (or other suitable unloading point), often miles away, and then transferring it onto a grab lorry for transportation to the landfill site.
Now, we explore alternatives to landfill disposal. Composting off-site is a more economical option compared to landfill, although it still requires handling and transportation, which poses environmental challenges. The process involves separating organic material from inorganic material, such as plastics and other rubbish, which are disposed of separately. Despite these challenges, the financial savings are significant, exceeding 50% compared to landfill. Putting weed to bank is another method we employ, which involves collecting small quantities of aquatic weed and placing them directly on the bank, typically on the offside rather than the towpath side, allowing them to compost naturally in situ.
Alternatively, larger quantities of weed can be deposited into a grab hopper and transported a short distance to a suitable site where we have sufficient space for deposition. In addition to reduced handling, transport, and related costs, putting weed to bank provides an opportunity for aquatic life inadvertently collected with the weed to find its way back into the water body. This includes young dragonflies, pollinator insects, frogs, and newts that may be present in or near the weed. As soon as the weed is placed on the bank, its high water content causes it to dry and compost rapidly. We can repeatedly deposit additional weed in the same locations over a short period. This differs from leaving weed in hoppers for a few days, where it starts to decompose, especially during summer, emitting an unpleasant odor.