Disposal or transplanting of growing media: environmental fate considerations for pesticide registrations in Great Britain and Northern Ireland

When you are applying for registration of a pesticide in Great Britain or Northern Ireland, for a product to be used on container grown plants, you must consider the potential for environmental exposure to occur. Exposure can occur as a result of disposal of spent growing media to land. Planting out of container grown plants can also result in environmental exposure. Both routes of exposure are considered in detail below. You need to assess exposure in soil, groundwater and surface water via drainflow following disposal or planting out.

Further information, including a PDF with a worked example of the disposal of growing media is provided in Protected uses: environmental fate guidance for pesticide registration in GB and NI.

If you require further guidance please contact HSE.

Disposal of growing media: First tier:

Products intended for use in containers often provide application rates in g a.s./m3 of growing media.  You must convert the application rate to g a.s./ha.  Assuming 50 tonnes of compost with a bulk density of 0.4 kg/L is applied per hectare this equates to 125 m3 of compost per hectare. For the first tier PEC calculations crop interception is set to 0 %. Based on regulatory precedents it is assumed that 37% of spent growing media contains no pesticide. 

Example calculation:  A PPP providing 40g a.s./m3 compost, will result in an application rate of 3.15 Kg a.s./ha (based upon 125 m3 compost/ha and 37% of growing media containing no pesticide).  

Planting out of treated pot grown plants

For details on pot grown plant groupings refer to Ornamental crops: Environmental fate considerations for pesticide registration in GB and NI.

First tier calculations should be based on the assumption that container grown plants are planted out immediately after treatment as a simple worst case.

Because the growing media is transplanted into soil the calculations are based upon a standard soil density of 1.5 g/cm3.

To calculate the PECsoil value following planting out, the following information is used:

Pot volume

Pot depth (cm)

Planting density

Plug plant 125mL

5

25 plug plants /m2
(plug plant volume of 125 mL (5x5x5 cm)).

1 litre

10

four medium sized plants grown in 1 L pots /m2

4 litre

15

one large plant grown in a 4 L pot /m2

The PECsoil in one hectare (1 hectare = 10,000 m2) based upon the planting depths, is considered in relation to the volume of growing media per pot and the number of pots over a hectare.

Pot size

The volume of soil per ha (m3)
based upon planting depth

Volume of media in each pot size (m3)

The number of pots planted per hectare

Plug plant

5 cm = 500

0.000125

250 000

1 litre

10 cm = 1000

0.001

40 000

4 litre

15 cm = 1500

0.004

10 000

Example calculations for a 1 L pot are detailed below assuming an application rate of 0.2 g/m3:

Based upon the volume of media within a 1 L pot and the number planted per hectare the total amount of treated media to be added to a hectare will be 0.001 m3 x 40 000 = 40 m3.

The total amount of active substance in the treated media can be calculated by multiplying the volume by the application rate, that is 40 m3 x 0.2 g/m3 = 8 g /ha.

Should this approach need refinement, consider degradation of the active substance in the media post treatment and before planting out. Appropriate intervals from treatment to planting out are 180 days for pot plants and hardy nursery stock, and 90 days for bedding plants. Also consider the presence of metabolites in compost at the time of planting out should also be considered.

Further information is available in the ADAS report of Disposal of growing media.

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Updated 2021-04-12