top of page


Woolmer Green Pond


The village pond in New Road is not particularly old in historical terms; it was constructed some time between 1837 and 1881. Prior to 1837 there were three ponds in Woolmer Green; one at Mardleybury Farm, another at the southern end of the site of what is now Carvers Croft, referred to as the Black Pond, which had been filled in by 1925, and another in New Road but on the opposite side of the road to where it is now.

It is not known exactly why the pond in New Road was filled in and reinstated its present location after 1837, but most of the original cottages in the road were also built after that date (except for the Fox pub building which is present on the 1837 tithe map and demolished in 2016) so presumably a land owner at the time decided to sell or redevelop the area. Perhaps the present New Road pond was constructed to compliment a cottage that faces on to it.

Prior to 2000 Woolmer Green was part of Welwyn Parish and in 1913 Welwyn Parish Council considered “whether the matter of cleaning out the pond at Woolmer Green does not come under the Public Health Act” and referred the matter to Welwyn Rural District Council so it looks like the pond was not in a healthy condition back then. Again in 1921 Welwyn Parish Council resolved that due to “the cost of cleaning out the pond (as per estimates submitted) the Clerk should write to the Rural District Council to ask the latter to undertake the work which it did on a previous occasion”.

By 1968 Welwyn Rural District Council recognised that the pond and surrounding land in New Road did not have a registerd owner and applied for the area to be designated as Common Land. This was undisputed and became final in October 1970.

Up to the 1990s there was only native yellow flag iris growing in the pond and the lack of oxygenating plants encouraged the growth of algae which caused green and red bloom to discolour the water. Countryside Management were consulted and with their expertise and funding from Shell Better Britain, the pond was dredged. In the later 2000's concerns were raised that the pond edge was eroding towards New Road so the edge of the pond was reinforced with coir rolls, goat willows were planted and a range of native plants were introduced and continue to thrive.


As the common land has no owner no one has the authority to carry out any work on it but it is protected under the Commons Registration Act of 1965 and the Commons Act 2006. The 2006 Act states that any local authority may “take any steps to protect the land against unlawful interference that could be taken by an owner in possession of the land.”  In the case of Woolmer Green pond and surrounding land, the Parish Council is the local authority that can protect the common and can implement minimal work to do this. Any work is always discussed and agreed with Herts County Council Countryside Management Service.

Canada goose and goslings


In 2014 it was discovered that Crassula helmsii (New Zealand Pygmy Weed) was present in the pond. It is very likely this has been introduced inadvertently by the public who have deposited unwanted goldfish in the pond. It should be noted that releasing non native species into the wild is an offence under the Wildlife and Countryside Act 1981. Hundreds of goldfish have now bred in the pond and although a spectacle to look at they will eat tadpoles, small frogs, invertebrates and insect life, which reduces natural biodiversity.

Crassula helmsii

Crassula helmsii was introduced into England from Tasmania in 1911, and sold throughout the 1920s as an ‘oxygenating

plant’ in the aquarium trade and by the 1950s was discovered growing in the wild. It was still being sold as a pond plant as late as the 1990s and continues to spread into waterways throughout the British Isles and across Europe and the US.

Crassula helmsii is recognised by the government as a non native invasive species and is listed under schedule 9(2) of the Wildlife & Countryside Act 1981 to which section 14 of the Act applies, this means that to cause it to be spread into the countryside is an offence carrying a fine of up to £5,000 or 2 years imprisonment. These penalties reflect the immense problems infestation of the plant causes and of the difficulties with controlling the plant.

Once the presence of Crassula helmsii was discovered in Woolmer Green pond the Parish Council immediately sought advice from the Herts County Council Countryside Management Service, the Environment Agency, The Centre for Agriculture and Bioscience International (CABI), Defra, the Centre of Ecology & Hydrology, the Freshwater Habitats Trust, Natural England, Hampshire Countryside Management and other Parish Councils with the same problem.

The Parish Council has also studied and reviewed the results of trials from various parts of the country. An excellent scientific example is the experimental trials undertaken in the New Forest. Here a range of ponds were selected where varying approaches in controlling Crassula were tried.



Woolmer Green pond in winter

Mechanical removal of the plant is out of the question as broken fragments 5mm in size can rapidly proliferate and will float off and establish new colonies making the problem worse.

The only herbicide that can be legally used in water is glyphosate but this can only be applied to dry weed occurring above the water line. As there is more of the plant under the water than visible on the surface herbicide application is limited. In the New Forest trials the majority of ponds tested were those that naturally dried out exposing the Crassula sufficiently so that it could be treated. The study states that “herbicide treatments could only be used to eradicate C. helmsii in ponds which dry out completely”.

If permanent ponds are to be treated it is essential that the herbicide is applied when the water level is at its lowest to obtain the greatest benefit from an application and this is where trials have run into problems. Inevitably in some years water levels have not been low enough for a herbicide treatment to be worthwhile. For herbicide to have any effect in Woolmer Green pond the water would have to be drained every year and this would be counter productive as the wildlife in the pond would be destroyed as well as the operation being prohibitively costly.

Treatment will not eliminate the weed completely and any submerged weed that remains can quickly regrow to pre-treatment levels and this has been known to occur in a matter of months. Another important consideration is that glyphosate may destroy native plants as well as the target plant and Crassula helmsii will out compete native plants in re-colonisation.

Disposal of waste is also a problem as any weed that is removed following treatment should ideally be disposed of on site by burying which is not practical on the small area of land surrounding Woolmer Green pond. Burning may not be permitted but in any case the waste would have to be dry. Leaving waste material on site is a risk, it could be tampered with or washed into the road and picked up by passing vehicles and spread into the countryside. Mardleybury Farm pond is unaffected by Crassula helmsii; it would be irresponsible to put it at risk and have two ponds infected.

The Hampshire Trials showed that herbicide treatment was not necessarily linked to a reduction in weed growth and that even with no treatment levels of the weed will fluctuate over time. Therefore the the growth of Crassula is complex and various natural influences affect its growth.


In November 2015 the Parish Council had to decide what to do, if anything, about Crassula helmsii in the village pond. It discussed a number of issues such as:

  • Should the Council spend public money over many years on chemical treatment that, due to inconsistent application or other unforeseen factors, will not produce the desired outcome?

  • Should the Council carrying out extensive work to land it does not own which could be contrary to the Commons Act 2006?

  • Should the Council go against  advice of its principle authority; the County Council’s Countryside Management Service; as well as national guidance and the results of countless trials which recommend no action?

  • Should the Council risk spreading Crassula into the countryside and endangering Mardleybury pond which could contravene s14 of the Wildlife & Countryside Act 1981?

  • Should the Council introduce annual applications of chemical herbicide into the pond without being clear of the long term effects on the natural flora and fauna?

  • Should the Council embark on a long term strategy without the support, guidance and expertise of the Countryside Management Service?


The Parish Council concluded it should follow national guidance and leave the weed undisturbed.


The reason for the success of Crassula helmsii in many parts of the world is because no natural predator has followed it’s migration to enable it to be kept in check. Since 2011 Defra has been funding a study to find a biological control that could be released in the UK.

The Centre for Agriculture and Bioscience International (CABI) has been leading the research and, along with the University of Tasmania, has conducted surveys throughout the plant’s native habitat in order to identify natural enemies that could be considered as biocontrol agents. Some of these were brought back to the UK for further study and a small mite; Aculus sp. (Eriophiyidae), was found to be species specific and able to reduce the plants reproductive success. In 2018 the mite was released in locations in the UK to further monitor its effectiveness and the Parish Council continues to follow progress as the control, in time, is likely to become commercially available.

In the meantime the abundance of beneficial native plants in the pond are vitally important as they provide competition for the Crassula and help to keep it suppressed.


In 2021 a company, working on behalf of a developer, carried out an eDNA test on the pond to ascertain if the pond contained great crested newts. Once the test was complete the company informed the Parish Council that the result was positive, Great Crested Newts were present. The Council asked the company if they could reveal who commissioned the investigation and to forward it a copy of the eDNA Report. The company said the report was related to a proposed development in Woolmer Green but could not provide further details or supply the report as it belonged to their client.

eDNA analysis is accurate but the Parish Council has not seen first hand evidence.

bottom of page