WEEDS

WHAT IS A WEED?

Any plant growing where it is not wanted

What is a weed?

“A plant whose virtues have not yet been discovered.”

Ralph Waldo Emerson

Detrimental Effect of Weeds

Look unattractive In pots in nurseries

In garden displays

In lawns

May make harvesting difficult Stinging nettles

Vine like weeds in harvesting machines

Allelopathy Toxic exudates from weeds that affect crop plants

Detrimental Effects of Weeds

Competition Weeds compete for surface nutrients Compete for bees Compete for water Compete for light

Poisonous or painful To people and animals due to chemical or physical defences

e.g. thorns

Harbour Pests and Diseases Humidity increased Over-wintering sites/alternative food source

Detrimental Effects of Weeds

Disrupt and destroy native eco-systems by competing for light, water, nutrients, physical space etc

Approximately 8 species of exotic (i.e. non-native) plants naturalise in NZ per year

Of those, approximately 10% will become invasive

Worst weeds around Dunedin?

In order of weed score rating#1 worst ecological potential= bomarea

then

old man’s beard hawthorn blackberry gorse sycamore banana passionfruit

Followed by…

buddleja ivy Darwin’s barberry broom cotoneaster convolvulus Chilean flame creeper

Benefits of Weeds

Add interest/colour to landscapes Increase diversity Edible/herbal remedies Act as subsoilers Access nutrients lower down in the soil profile Stabilise hillsides Act as indicators Act as a green manure Contain beneficial chemicals Provide food for predator adults Are a food source for a range of wildlife Provide shelter

Benefits of weeds whilst growing

Weed Examples Benefits

Knotgrass, hedge, bindweed, dandelion, poppy, teasel, thistle, chickweed, shepherds purse, plantain

• Are a food source for a range of wildlife

Birds: seed.

Field bindweed, bramble, thistle, ivy, da ndelion, campion, medick, groundsel.

Butterflies: nectar

Thistle, fat hen, medick, ivy, campion, groundsel, clover, white dead -nettle

Bees: nectar

Campion Moths: nectar

Dock, sorrel, nettle, groundsel, ragwort, burdock, charlock, bindweed.

Caterpill ars: foliage

Sun spurge, fumitory Ants: seed oils

Nettle, fat hen, medick, dandelion, nipplewort, bindweed

Predators attract ladybirds and hoverflies whose larvae eat aphids.

Creeping thistle, ground elder, nettle, shepherd’s purse, chickweed

Beetles

Benefits of weeds whilst growing

Weed Examples Benefits

Chickweed, clover, vetch • Can act as a green manure Provide ground cover. Protect soil surface from heavy rain. Encourage soil life. Dig the soil with their roots. Prevent leaching of nutrients. Inc rease fertility. Control erosion. Improve microclimate.

Corn cockle increase the yield of wheat, nettles enhance the flavour of herbs

• Contain beneficial chemicals

To enhance the growth of other plants

All weeds • Increase diversity Fewer pests. Tap the soi l for different nutrients.

Ivy • Provide shelter: Cover for birds. Hibernation for butterflies

Daisy, toadflax, red dead nettle, campion, comfrey, poppy.

• Are attractive Flowers provide colour in the garden

Dock, dandelion, comfrey • Act as subsoilers Bring up nutrients from the subsoil. Improve structure and drainage of soil

• Act as indicators Different weeds grow on different types of soils

Indicator Weeds

Weeds Indicate Comment

Field pansy, field mouse -ear, poppy, charlock

Calcareous soil Test pH. No liming required. Acid -loving plants will need to be planted in acid compost.

Corn spurrey, sorrel, plantain, parsley peirt.

Low pH (acidic) Test pH.May require liming.

Chickweed Neutral soil. Just right!

Cleavers, red dead -nettle, chickweed, borage, creeping thistle, sun spurge, fumitory, charlock, speedwell, redshank.

High in nutrients. Use organic matter to maintain fertility.

Groundsel, stinging nettle, fat hen, ground elder, dock, sowthistle, chickweed.

High nitrogen Plants may be sappy. Leafy vegetables will thrive.

Shrubs, perennials and annuals may flower less. Balance nutrients (with seaweed compost)

Indicator Weeds

Weeds Indicate Comment

Increase in weeds that flower in summer/autumn.

Declining fertility/neglect. Add organic matter. Sow green manures. Recycle nutrients. Maintain ground cover.

Clover, medick, vetch. Low nitrogen. Grow nitrogen -fixing green manures (tares, clover)

Silverweed, greater plantain, pineapple mayweed.

Compact soil. Sow green manures. Make deep beds. Cover soil. Add organic matter. Double dig..

Creeping buttercup, horsetail, silverweed, coltsfoot.

Poor drainage. Aerate soi l. Make deep beds. Sow green manures. Add organic matter. Double dig.

Speedwell, fumitory, chickweed.

Well-aerated, moist soils. Just right!

Spurges, chickweed, groundsel, fat hen, dandelion.

High in humus. Just right!

Characteristics of a weed

Colonisers of disturbed habitats Reproduce very quickly Huge genetic diversity Seed dormancy Habit/Form Habitat Defence system Resistance to herbicides A wide range of dispersal mechanisms

Reproduction Methods

Rhizome Tubers Root or Shoot Segments Bulbs Seeds Stolons

Weed Lifecycles

Annual Biennial Perennial Ephemeral

Annuals

A plant that completes it’s lifecycles in one calendar year or growing season.

Seed Germination Growth Flowering/reproduction Seed set Death

Annual weeds ensure survival by seed production.

E.g. fathen, groundsel, cleavers

Biennials

Year 1 Grow from seed and develop strong root systems and cluster of leaves

Year 2 Mature, produce seeds and die

Biennial weeds ensure survival by seed production

E.g. brassicas, shepherd’s purse

Perennial

A plant that persists and produces reproductive structures year after year.

Reproduces by Seed and Vegetatively.

taproots

rhizomes

creeping stems

bulbs/corms

E.g. dock, creeping buttercup, couch, oxalis

Ephemeral

A plant that completes many lifecycles within one calendar year or growing season. Ephemeral weeds are very short lived.

Seed Germination Growth Flowering/reproduction Seed set Death

Within weeks

Ephemeral weeds ensure survival by seed production.

Implication for weed control

Suitable control measures Lifecycle stage

Annual/ Mowing, cultivation or contact herbicide.

Ephemeral Before flowering.

Biennial Cultivation or contact herbicide.

Preferably in the first year of growth or in the second year before flowering.

Perennial Individuals removed, systemic herbicides.

Preferably before maturity – in any year before flowering.

Remember the saying:

“One year’s seeding

means seven year’s weeding!”[1873 Harland & Wilkinson Lancashire Legends 190]

Weeds – adaptations and survival

Plant type (life cycle) Plant vigour/Growth rate Weed habit Seed production Seed dispersal Seeding height Physical and chemical defences Root and specialised underground storage systems Herbicide resistance

Weeds – distribution and growth

Different environmental conditions favour different patterns of weed distribution and growth.

Coastal areas Amenity turf Bush/woodland areas Wetlands Wasteland Garden bed/borders

Categorising Weeds

Dicotyledons

- Plants which produce two seed leaves. Includes broad leaved weeds

Monocotyledons

- Plants which produce a single seedling leaf and includes grasses and cereals

Many herbicides are selective in their mode of action. I.e. they only kill a certain type of plant

Dicotyledon

Source: University of Californiawww.ipm.ucdavis.edu

Monocotyledon

Monocotyledon versus Dicotyledon

Broad leaves Net-like pattern of

veins in leaf Flower parts in

multiples of 3 One cotyledon

Long, narrow leaves Parallel pattern of

veins in leaf Flower parts in

multiples of 2, 4 or 5 Two cotyledons

Weed Management

Cultural control Biological control Chemical control

Legal methods to enforce the 3 methods above

Combined together =

Integrated weed management

Cultural Weed Control

Prevention

Keep surrounding areas weed free Mow before flowering General hygiene - machinery, tools, mulches Don’t buy in with mulch, compost or plants

CultivationHand weeding, hoeing, cultivation.Cheap, time consuming.annual/perennial seedlings

Heat treatmentwater or heat/fireannual/perennial seedlings

Mulches (organic and inorganic)Suppresses weed growth by eliminating lightNo light = No weed

Physical control of weeds

Stale Bed Technique

Preparing the bed in advance so that the weeds germinate and can be removed before you sow your crop

Digging

Turn the soil over and bury the weeds upside down - Instant Gratification

- Digging when the soil is wet may destroy the soil structure and cause compaction

- Some weeds may be slow to decompose and go slimy - don’t replant too soon

- Digging will bring weed seeds to the surface- Try digging in the dark!

Mulching

Mulches

Prevent weeds germinating - excludes light/unsuitable surface

Conserve soil moisture - reduces evaporation Prevents rapid fluctuations in soil temperature Reduces winter heat loss from soil (can prevent them

from warming thus increasing frost damage) Prevents soils splashes - reduces soil borne fungal

diseases Reduces erosion caused by rain run-off (can prevent

water getting into the soil)

Organic Mulches

Sawdust Bark chips Peat Hay Straw Leaf mould Spent mushroom compost Fowl manure Stable manure Grass clippings Wood shavings Newspaper

Inorganic Mulches

Black polythene Woven plastic fabrics/’Weedmat’ Gravel Scoria Pebbles Rocks

Biological Control of Weeds

The use of insects or diseases to control weeds

Biological control does not aim to eliminate the weed but to reduce it to a level where it can be effectively controlled by other methods

Biological Control of Weeds

Weeds Weeds are mostly introduced (accidentally or deliberately) They often flourish in NZ because:

- of the more favourable climate

- insects and disease which naturally control them are not here

Biological control Biological control can be initially slow to show any results.

However once it is established it costs nothing. The insect damages the host plant only, therefore there are

minimal environmental side effects.

To control gorse: gorse weevil (Apion ulicis) and gorse spider mite (Tetranychus lintearius)

To control Californian thistle: leaf beetle (Lema cyanella)

To control old man’s beard: old man’s beard leaf miner (Phytomyza vitalbae)

Examples of Biological Agents

Selection of Biological Agents

Scientific testing Host specific MAF permission Quarantine Breeding up Release Assessment

Chemical control of weeds

How Herbicides Work

Block a Fundamental Growth Process

- destroy chlorophyll

- stop cell division

- stop respiration

Cause stunting, unnatural growth, and dry out the plant (desiccant)

Modes of action

Contact Herbicides

- foliar applied sprays

- enter leaves by diffusion entering the xylem

- kills the plant at point of spray contact

- use on annuals and young perennials

e.g. Universal Weed Killer, Preeglone

Systemic or Translocated Herbicides

- absorbed either through the roots or leaves

- the chemical is then translocated or moved about within the plant as it grows

e.g. Roundup

Herbicide Selectivity

Selective Herbicide

- Kills the weeds without harming the desired plants

e.g. Lawn weedkillers

kill broadleaves and leave grasses

e.g. Grasskiller

Non-selective herbicides(broad spectrum)

- Kills all vegetation for total control

- Do not use on lawns

e.g. Amitrole, Round-up

What makes a herbicide selective?

Herbicide selectivity:

Can be a feature of the herbicide

- how it is made, active ingredients, different chemical reaction to different types of plants

The way we apply it i.e. Timing

Timing of herbicide use

Pre-planting

- Spray before planting or sowing.

- Target is germinating and existing weeds e.g. Round-up

Pre-emergence

- Spray to control weeds before weeds/crop emerges

- Weed free seed bed and reduced competition

e.g. Foresite

Post-emergence

- Spray herbicide after weed/crop emerges

- True selective e.g. Yates Grasskiller

Soil acting herbicides

Soil acting herbicides are absorbed by weed shoots as they emerge through the soil resulting in weed death

Soil acting herbicides that stay in the soil and continue to control weeds for a extended period of time are called ‘Residual Herbicides’

Prefix D 12 months

Foresite 3 months +

DAS 12 months

Integrated Weed Management

Using a combination of control methods to get the best results

- Cultural

- Biological

- Chemical

Using minimal amount of herbicides Using herbicides safely Promoting plant health Understanding weed lifecycles to enable effective control

Weed management - Example

Poor lawn management gives weeds a competitive advantage.

Encourage a healthy lawn- regular mowing (little & often)- regular fertilising- Water in dry conditions- Scarifying- Pest & disease control

Discourage weeds- choose good quality lawn seed- control weeds in surrounding areas- don’t let weeds flower- don’t buy weeds in

Control existing weeds using a combined approach

Weed Books

Weed books on close reserve at the Bill Robertson Library

Taylor, R. L. (1981). Weeds of roadsides and waste ground in New Zealand. Nelson: Taylor

Uprichard, E. A. (1985). A guide to the identification of NZ common weeds in colour. Palmerston North: NZ Weed and Pest Control Society.

Taylor, R. L. (1983). Weeds of crops and gardens in New Zealand. Nelson: Taylor

Parham, B. E. V; Healy, A. J. (1976). Common weeds in New Zealand. Wellington: D.S.I.R.

References

University of California www.ipm.ucdavis.edu