Those who raise nursery stock from seed — from rootstocks and forest trees to hardy herbaceous plants — look with envy at the rapid developments in seed quality and propagation techniques that have taken place in the bedding and pot plant sectors.
Although there has been a shift in the bedding industry to cuttings for some newer cultivars, seed remains the starting point for more than 80 per cent of the crop. And improvements in seed quality mean that crops such as Aubrieta — mostly propagated from cuttings 15 years ago — are now regularly propagated from seed.
Pre-germination treatments mean seed companies in the bedding plant market now offer seed not by the kilo but in specific numbers for germination in cell trays, and propagators will expect almost 100 per cent germination. With such good quality seed comes the ability to automate sowing and transplanting or potting-on to a far greater extent than can be achieved with vegetative propagation — despite the inventive use of industrial robots in the Netherlands for taking or striking cuttings.
The size of the nursery stock seed market compared with the worldwide bedding plant industry, together with the wider diversity of species and their often more complex requirements for cleaning, sorting and pre-germination treatments, have held back major innovation in nursery stock seed quality. But there have been some recent developments in treatments to improve the quality of commercial woody plant seed lots. As growers at a recent International Plant Propagators’ Society (IPPS) conference in Belgium heard, new techniques to improve dormancy breaking treatments have made possible more accurate seed sorting and pre-treatment with fungicides.
Nursery stock growers have traditionally used cold stratification — keeping seed cold but not frozen in moist sand or peat in a fridge, cold store or cold frame — as a standard treatment to break seed dormancy in a wide range of temperate woody species. But, as Dutch nurseryman Niels Dictus told the conference, stratification can be difficult to control: "Not all seeds will have broken dormancy after the eight weeks or so of standard treatment, while others will have begun to germinate prematurely, before they have been sown," he said.
Dictus runs a business in the Zundert area of the Netherlands, offering seeds and seed treatment services to tree nurseries and producing plug-grown nursery stock seedlings. He uses a modified seed stratification technique based on controlling the moisture content in the seeds. The aim is to reduce premature germination while increasing the proportion of seeds that go on to break dormancy and germinate after sowing. "It allows the seed to be stratified for much longer, for example 16 to 20 weeks in the case of Acer platanoides and 16 to 18 weeks for Fagus sylvatica," he said. "The longer stratification period results in much faster germination after the seeds are sown, while controlling the moisture content prevents premature germination."
Dictus said 15 weeks’ normal stratification achieves 34 per cent germination in A. platanoides, but lengthening the period to 19 weeks brings more seeds out of dormancy and increases germination to 78 per cent. In F. sylvatica, 16 weeks’ cold treatment results in 82 per cent germination compared with 54 per cent after 10 weeks.
Controlling the moisture content of the seeds depends on knowing their moisture content to begin with. This means taking a sample from the batch and weighing it before drying in an oven (17 hours at 105°C). The difference in weight, as a percentage of the original weight, is the moisture content (see panel). You can then work out how much water, by weight, to add to the main batch in a plastic bag to bring it to a specific moisture content (assuming the seeds absorb all the water you add).
For example, said Dictus, the optimum moisture content for A. platanoides is 36 per cent. "The bags of seeds can be stratified at 4°C but you must check the moisture content every week: in the Acer, 34 per cent is too low, 38 per cent is too high and will cause premature germination." For F. sylvatica, Dictus said the optimum moisture content is 30 per cent, with 28 per cent too low and 32 per cent too high.
Ria Derkx, a researcher from Applied Plant Research at Lisse, the Netherlands, said the longer stratification period enabled by the controlled moisture technique means seeds germinate uniformly over a wider range of germination temperatures. "Optimum moisture content and controlled moisture content treatments have been developed for about 20 species of broad-leaved trees and conifers so far," she said. The stratification period for some species, such as Larix kaempferi, is so short there is no time for seeds to germinate prematurely, making a controlled moisture regime unnecessary.
"Being able to control premature germination has led to the development of new ways of improving seed-lot performance, such as liquid sorting for tree seeds," Derkx said. Liquid sorting is a technique developed by the vegetable seed industry and a refinement of the idea of floating seeds in water to separate live from dead seed.
"The seed’s specific density is related to its germination behaviour," said Derkx. "By testing the seeds in liquids of different densities, in which they will either float or sink, we can sort them according to their requirement for different pre-treatments."
Seeds are liquid-sorted in a large funnel-shaped tank which can be filled from the bottom with four different solutions, starting with the lowest density. The seeds that float are separated off, the first liquid emptied and the next pumped in, and so on. The germination of each seed fraction is tested and the propagator can then decide which pre-treatment to give; fractions deemed likely to germinate poorly can be discarded. The specific gravity of the liquids can be fine-tuned to the optimum specific densities of each species by tweaking the temperature and the concentration of the solutions. The liquids used include water (specific density 1.0g cm2), salt solutions (>1), ethanol (0.79), ether (0.66), glycerol (1.26) and other organic solvents.
In general the lightest (lowest density) seeds in a batch tend to have the poorest germination, although sometimes the heaviest fraction germinates poorly, too. By removing these fractions, the performance of a seed lot can be improved for the grower, because a higher proportion of what is sown is likely to germinate and the resulting crop is likely to be more uniform.
In Derkx’s trials, most species have been successfully liquid sorted either before or after dormancy breaking. "For example, in Larix we found an unsorted seed batch would typically have 40 per cent germination," she said. "By removing non-germinable and poorly germinating fractions you are left with seeds that can be pre-treated and sown to give 80 per cent germination." Derkx said trials had been undertaken so far with Acer pseudoplatanus, Carpinus betulus, Crataegus monogyna and Prunus avium as well as larch.
One potential problem with liquid sorting is that being immersed in a liquid can raise the seed moisture content, thereby increasing the risk of premature germination.
Derkx also pointed out that while laboratory germinations were consistent, field emergence could be more variable — for example, in trials in 2000, 57 per cent of sorted Larix kaempferi seed emerged after field sowing compared with unsorted; in 2002, emergence was 32-per cent from the unsorted seed lot and an only slightly improved 36 per cent after sorting.
Some of the variability in field emergence on nurseries is down to a range of soil-borne fungal diseases that attack the seed and the emerging seedling. A second advantage of the controlled moisture content stratification treatment is that it makes it possible to film-coat tree and shrub seeds with fungicides.
Derkx told the IPPS conference about trials in which a range of tree seeds had been film-coated with fungicides after dormancy breaking. "We tested both for the possibility of phytotoxic effects on the germinating seedlings and for effectiveness at protecting the seedlings from disease," she said. The fungicides included fosetyl-aluminium (Aliette, 3 or 6g per kg seed) and iprodione (Rovral Aquaflo, 3 or 6ml per kg seed); and a combination of propamocarb hydrochloride (6 or 12ml per kg seed) and thiophanate-methyl (4 or 8ml per kg seed).
In the field germination tests, only the Aliette treatment was phytotoxic (on Fagus sylvatica). Previcur effectively protected Fagus sylvatica from damping-off, while both tested combinations protected Acer palmatum. Unfortunately, said Derkx, there was too little damping off, even in untreated plots, to draw firm conclusions and further trials are under way.
While controlled moisture stratification could be readily adopted by nurseries growing trees, shrubs or herbaceous perennials from seed, liquid sorting and film coating require specialist equipment. But if these techniques were more widely adopted by seed suppliers, it could significantly improve the quality of seed available to nurseries. In turn, that could make for greater opportunities to mechanise or automate seed propagation for hardy plants.
Moisture controlled tree seed stratification
• Weigh seeds into plastic bags, taking a sample of each batch to calculate the initial moisture percentage.
• Weigh the sample and then dry in an oven to remove moisture. Weigh the sample again and use the difference in weight to calculate the moisture content as a percentage of the weight of the seed batch. This figure is used to work out the amount of water to add to the batch to maintain a given moisture content.
• The optimum moisture content is critical. Two per cent too low can cause the seeds to dry out, while two per cent too high can trigger premature germination.
• Add the required weight of water and mix with the seeds in the bag. For example, if 100g of seed contains 30g of water, the moisture content is 30 per cent. If the optimum moisture content for that species is 36 per cent, then 6g of water per 100g of seeds must be added.
• Place the bag of seeds in a refrigerator or cold store.
• Re-mix and check the moisture content of the seeds being treated every week.