Weeds compete with cultivated plants for resources like water, nutrients, and light. They can reduce crop yields and the aesthetic appeal of ornamental plantings. Weeds can also act as hosts for pests and diseases. There are three main types of weeds: ephemeral weeds which have multiple generations per year, annual weeds which complete their lifecycle in one year, and perennial weeds which live for many years. Weed control methods include physical removal, herbicides, and cultural practices. Herbicides can be contact, systemic, residual, or selective depending on their mode of action.

1. RHS Level 2
Certificate
Plant Health Week 17 – Weeds,
identification and control.

2. Learning Objectives
2. Understand the problems posed by weeds in horticulture and
how these problems can be minimised.
2.1 Describe what is meant by a weed.
2.2 Describe how weeds reduce crop productivity and reduce the
visual appeal of plantings for display.
2.3 Describe the role of weeds as alternative hosts for plant
pathogens.
2.4 Describe the biology of ephemeral, annual and perennial weeds.
2.5 State the botanical names of TWO examples of EACH of the types
described in 2.4.
2.8 State what is meant by contact, residual, translocated and
selective herbicides, and describe ONE situation where EACH type
would be used appropriately.
2.9 State the active ingredient of ONE example of EACH type of
herbicide described in 2.6.

3. What is a weed?
 Any plant in the ‘wrong’ place.
 However some plants are generally
undesirable in cultivated ground
 Some weeds are serious invasive threats to
the environment e.g. Fallopia japonica but
most simply compete with cultivated plants
for water, nutrients and light.

4. Why are weeds a problem?
 Compete for water and nutrients with
desirable plants.
 Shade seedlings and plants – compete for
light.
 Are unsightly and reduce ornamental effect.
 Reduce yields of crops
 Act as alternate hosts for pests and diseases.

5. Weeds as hosts for pests
and diseases
 Weeds can provide pests such as aphid with a
place to overwinter – leading to rapid
infestation of cultivated plants in spring.
 Weeds can also host viruses and fungal plant
diseases that can then be transmitted to
cultivated plants.
 Removing weeds can break the life cycle of
the pest or remove the source of disease and
therefore reduce the chances of plant
problems arising

6. Types of weed
 Ephemeral – a number of generations
produced from seed to seed each year.
 Annual – germinates, grows, flowers, sets
seed and dies in one year.
 Perennial – live for a number of years,
growing and flowering and setting seed
each year.

7. Ephemeral weeds
 Cardamine hirsuta – Hairy Bitter
Cress; Senecio vulgaris – Groundsel.
 Produce vast amounts of seed that
remains viable in the ground for a
number of years.
 Shallow rooted and very quick
growing.

8. Senecio vulgaris
Groundsel

9. Annual weeds
 Chenopodium album (Fat Hen);
Gallium aparine (Cleavers,
Goosegrass)
 Shallow rooted and quick growing
 Reproduce by seed and die after
setting seed.

10. Fat Hen
Chenopodium
album

11. Perennial weeds
 Aegeopodium podagraria (Ground Elder);
Taraxacum officinale (Dandelion) and Plantago
major (Greater Plantain)
 Reproduce by seed, but also vegetatively, which
makes them hard to eradicate.
 Live and grow for many years.
 Typically deep or extensive root systems, tap
roots etc.

12. Ground elder Aegopodium
podagraria

13. Means of control
 Physical – hand weeding, hoeing,
digging.
 Chemical – herbicides
 Cultural – mulching, weed fabric,
ground cover plants.

14. Herbicides
 Contact – kill what they touch e.g. diquat
 Systemic or translocated – taken up by the
plant, so they will kill all parts if enough is
absorbed, e.g. glyphosate
 Residual – usually combined with a contact
herbicide, these kill emerging seedlings, e.g,
diflufenican.
 ‘hormone’ or selective – kill broadleaved
weeds (dicotyledons) only, e.g dicamba.

15. Worksheet answers
1. Hand weeding or hoeing – trying not to turn the
soil over or damage the rows of cabbages.
2. Hand weeding – chemical control would be
difficult as the hedge could also be damaged.
3. Digging or careful spot treatment with
glyphosate whilst the perennials are still
dormant.
4. Dicamba – selective for broadleaved weeds or
a daisy grubber to dig up.
5. Very little (apart from growing a GM grass with
resistance to glyphosate).

16. Learning Outcomes
2. Understand the problems posed by weeds in horticulture
and how these problems can be minimised.
2.1 Describe what is meant by a weed.
2.2 Describe how weeds reduce crop productivity and reduce the
visual appeal of plantings for display.
2.3 Describe the role of weeds as alternative hosts for plant
pathogens.
2.4 Describe the biology of ephemeral, annual and perennial
weeds.
2.5 State the botanical names of TWO examples of EACH of the
types described in 2.4.
2.8 State what is meant by contact, residual, translocated and
selective herbicides, and describe ONE situation where EACH
type would be used appropriately.
2.9 State the active ingredient of ONE example of EACH type of
herbicide described in 2.6.