This group includes many arable weeds. The plant produces a large number of small seeds which are often adapted for long range dispersal, particularly by wind. Their small energy reserves are insufficient to enable the seedling to grow in competition with established plants. Thus many of these species have evolved germination requirements which ensure that the seed germinates only in open sites. Such sites are characterised by warm sunny days and cool dark nights. If established plants are present they reduce the amount of red light which reaches the soil surface and reduce the temperature fluctuations. Thus the seeds of many opportunist species require (red) light and fluctuating temperatures for germination. This strategy is common in members of the family Compositae such as Lactuca and Matricaria and in small seeded grasses such as Poa and Agrostis.

These species should be surface sown, with diurnal light and temperature fluctuation.

LARGE SEEDED COMPETITORS

A deeply dormant seed which germinates only after it has experienced an adequate cold period is an adaptation to severe winter conditions. This permits the species to overwinter as a relatively resistant seed in conditions which would kill the vegetative stage. The temperate species of Polygonum typify this strategy.

A strategy which prevents seeds germinating too early in the spring before the danger of night frosts has passed is a requirement for warm soil temperatures before germination will take place. Species of Xanthium and Amaranthus tend to germinate only at temperatures above 20 C.

A seed which germinated in the moist environment of the berry in which it matured might be digested by the bird which ate the berry. Thus seeds of plants such as Solanum spp. tend to contain inhibitors which prevent germination. These inhibitors may decline with time, may be leached out by substantial rainfall, or be removed by the digestive processes of the animal which eats the berry. HERBISEED leaches seeds from berries before drying and storing them.

Simple post harvest dormancy is alleviated by storing the dry seed for adequate time. HERBISEED normally stores seed which is known to have a significant time dependent post-harvest dormancy for one year before distributing it to our customers.

Another type of endogenous time clock occurs in some populations of Galium aparine. In seed of this species a short period of post-harvest dormancy is succeeded by several months of non dormancy before a second period of dormancy commences. Cycling between dormancy and non dormancy can continue for several years. These cycles are believed to prevent the seed germinating in summer when conditions are not conducive to plant establishment.

HERBISEED stores seed from species which are likely to show extended post harvest dormancy for at least one year before dispatching it to our customers. This is achieved with the seed dried to 10% moisture and stored in an insulated building at 30% relative humidity.

Seed which undergoes cyclical dormancy can be deep frozen when in its non dormant phase. When it is taken out of the freezer it will still be non dormant. If it is intended to store seed frozen for over one year, ensure that its moisture content is not more than 8% and seal the container to prevent it picking up moisture from the saturated air in the freezer.

There is some evidence that storage in a refrigerator at a temperature of 1-5 C reduces dormancy in certain species.

Sometimes called 'vernalisation', stratification subjects moist seed to cold conditions, thereby fulfilling the cold requirement of winter avoiding species. The method is as follows:

1 Mix the seed with twice its volume of coarse sand or vermiculite

2 Wet the mixture thoroughly

3 Drain for three hours but do not allow to dry out

4 Ideally leave for several days at room temperature and keep moist

5 Place in a thin plastic bag or unsealed container

6 Place in a refrigerator at between -4 C and 4 C

7 Periodically replace moisture lost by evaporation

8 Sub-sample seed at monthly intervals and test for germination at 15 C

An alternative is to sow the seeds in normal growing medium and put the sown containers outdoors in autumn or into a cold room. After an adequate period remove them from the cold and place in the glasshouse to germinate. Pre-sowing seed trays and putting them into a cold room for a week or two is effective in promoting the germination of many temperate weeds. However, many species of Umbeliferae will only germinate after experiencing a full winter outdoors.

Seeds which have an impermeable seed coat require the coat to be made permeable by physical scratching or treatment with solvents or acids. Scarification with sandpaper is the simplest method to use on small quantities of seed. The method is as follows:

1 Place a piece of coarse sandpaper over the bottom of a small tray

2 Place a single layer of seed on the sandpaper

3 Rub the seed with another piece of sandpaper or a sanding block

4 Stop when the seed is thoroughly scratched, before the seed coats break

5 Test for efficacy by putting the seed in warm water overnight. It should imbibe visibly, if does not, drain, surface dry and scarify it further.

Seed of plants adapted to arid conditions can sometimes be stimulated to germinate by hot water treatment. Effective treetments vary from 24-hour soaking in water at 60C to a few seconds immersion in near boiling water. Optimum treatments need to be found by experiment if seed of Mediterranean or arid climate species fails to respond to scarification.

Surface sow or cover with no more than 2mm of soil. Light is necessary, with light/dark periodicity and temperature fluctuations over the range 5-15 C an advantage for some species. Surface sowing requires constant attention to ensure that the soil surface does not dry out, even temporarily.

Surface sow in the light at 20-25 C. Ensure that the soil surface does not dry out even temporarily.

Abutilon theophrasti: Place seed in a tea strainer or sieve. Pour boiling water over it for 3-5 seconds. Allow to drain and sow 3mm deep. Germinate at 20-25 C.

Ambrosia species: Post harvest dormancy can persist for several months. Soak for 24 hours in 2-3 changes of warm water. Sow 5mm deep and germinate at 25 C in the light. Some seed batches respond to a brief cold period before germination.

Avena fatua: Most populations have post harvest dormancy which lasts up to three years. This can be alleviated by removing the seed from the caryopsis or by physically damaging it. Stratification and potassium nitrate treatment are partially effective. Sow 10mm deep and germinate at 5-15 C.

HERBISEED supply seed of a dormant population which has been stored for 2 years to allow the dormancy to decline. HERBISEED can also supply seed of a genetically non dormant population.

Bilderdykia [Polygonum] convolvulus: Stratify for three weeks, germinate at 10-20 C. Some batches may germinate without stratification.

Cassia species: Scarify strongly, soak overnight and repeat scarification if the seed does not imbibe. Sow 5mm deep and germinate at 20-25 C.

Chenopodium album: Individual seeds tend to germinate gradually over several weeks. This can be accelerated by removing the involucre from the seed and stratifying for 2 weeks at 5 C. HERBISEED supply seed from which the involucre has already been removed. Sow 2mm deep and germinate at 10-25 C in the light. Germination can be synchronised by sowing the seed, watering it, then placing it in a cold room at 5-10C for a few weeks before bringing it into the warm to germinate.

Cyperus iria: Surface sow, firm soil well and germinate in the light, at 20 C ensuring that the soil surface never drys out.

Datura species: Soak seed in several changes of warm water for 24 hours. Sow 5mm deep and germinate at 15-25C.

Fumaria officinalis: This species displays much batch to batch variation in germination. Light scarification plus up to three months stratification can be effective on batches which do not germinate readily. Sow 3mm deep and germinate with temperature cycling between 5 C and 15 C.

Galeopsis tetrahit: Scarify, then stratify for 8 weeks. Sow 3mm deep and germinate at 5-15 C.

Galium aparine: Sow 10mm deep, germinate in temperatures fluctuating from 10-20 C in the dark, ensuring that the the soil is watered to over field capacity at least once per day.

This species is particularly difficult to germinate in summer. Where it is required for year-round glasshouse screening it can be sown in pots or seed trays which are watered, then held for two weeks in a cold room for germination to occur before moving them to the greenhouse.

HERBISEED stock a population which originated in a German potato field which appears to germinate much later in the spring than most populations of the species.

Imperata cylindrica: Surface sow on free draining soil and keep very moist, preferably in a mist propagator. Keep at 25 C. Seedling establishment is slow.

Ipomoea species: Scarify seed and wash briefly with water. Sow 2-5mm deep depending on seed size and germinate at 25 C.

Juncus bufonius: Surface sow, firm well into the soil surface and keep the soil very wet, preferably under a mist propagator. Germinate at 10-15 C in diffuse light.

Kochia scoparia: Rub seed and blow away as much extraneous material as possible (HERBISEED seed is already rubbed and winnowed). Soak seed in warm water for 24 hours, surface sow in diffuse light and germinate at 20-25 C.

Polygonum aviculare: Remove papery husk around seed (seed sourced from HERBISEED has already had the husk removed). Stratify for 6 weeks. Sow 2mm deep and germinate at 10-20 C.

Polygonum lapathifolium, Polygonum persicaria: As for Polygonum aviculare, but some batches germinate without stratification.

Ranunculus spp. The embryo of several species of Ranunculus is immature when the seed is shed, and develops only after the seed encounters conditions suitable for germination. Thus germination occurs several weeks to months after sowing. This is true of R.. repens, R.. acris and R. arvensis. If relatively rapid germination is required use R. sardous, which normally emerges within two weeks of sowing.

Raphanus raphanistrum: Seed is enclosed in an indehiscent seedpod. Break seedpods into 1-2 seeded sections (already done in seed from HERBISEED). Soak in 2-3 changes of warm water or 2% potassium nitrate solution for 24 hours. Sow 5mm deep and germinate at 15 C.

Salsola Kali: Wear gloves to rub and blow away as much as possible of the spiny extraneous material (this has already been done in seed from HERBISEED). Sow 2mm deep and germinate at 25 C in the light.

Sesbania exaltata: Scarify, then soak in running warm water for at least 24 hours until the water runs clear. Sow 5mm deep and germinate at 25 C

Sida species: Rub seed and blow away as much of the husk as possible (This has already been done in seed from HERBISEED). Soak 24 hours in warm water. Sow 2mm deep and germinate at 25 C in the light.

Sinapis arvensis: Large differences occur between different seed batches of this species. Some are deeply dormant. Stratification for 2-3 weeks is often helpful. Sow 3mm deep and germinate at temperatures cycling between 10 and 20 C.

Solanum nigrum: Leach out soluble inhibitor by soaking in cold water for two weeks (this has already been done in seed sourced from HERBISEED). Sow 2mm deep, germinate at 15-20 C in the light. Do not allow soil surface to dry out.

Sorghum halepense: Remove caryopses (already completed in HERBISEED seed). Soak seed for 24 hours in running water at 30 C or in a well stirred water bath. Sow 3mm deep and germinate at 25 C in the light.

Veronica hederifolia: Some batches have a post harvest dormancy which is alleviated by stratification for up to 6 weeks. HERBISEED has recently obtained a population which appears not to have any dormancy. Sow 5mm deep and germinate at 10-20C.

Viola arvensis: Post harvest dormancy in this species may last up to a year. It can be alleviated to some extent by stratification. Seed sourced from HERBISEED has been shelf stored for at least one year and is non-dormant. Sow 2mm deep and germinate at temperatures ideally fluctuating between 10 C and 20 C.

CONCLUDING REMARKS

From a practical point of view, it is always easiest to ensure that the seed sample is as free of non-seed material as possible and sow it according to the general recommendations at the start of this section. Special treatments will only be necessary if this fails. It is well worth bearing in mind that a great many weed species germinate more readily in soil than in more uniform substrates, particularly filter paper. Furthermore, with many weed species, fluctuating temperatures tend to result in a higher percentage germination than constant temperatures.

A pragmatic solution to the complex interactions between seed batch, test conditions and time is to stock several batches of the same species, changing batch if one batch begins to germinate inadequately.