Rutherglen Research Institute, Rutherglen Vic.
1. What is Direct Sowing?
Simply, it is sowing seed of the desired species into the final location instead of raising them as seedlings prior to planting out.
2. Why Direct Sow?
• It is a lower cost method of establishing trees compared to planting seedlings.
• Root systems develop naturally and are not so prone to misshapen roots which often happens with tubed stock. Better root systems generally means trees survive better, grow better and are not so subject to windthrow.
• Large areas can be established quickly, with relatively low labour input.
3. Disadvantages of Direct Sowing
• Germination percentage of seed sown can be very low if conditions are not right.
• Adverse seasonal conditions may cause low survival. In some years the technique works better than others, and current research at a number of locations throughout Australia is directed towards developing more reliable techniques of direct seeding.
South Australia's Woods and Forests Department, a number of mining companies in Western Australia and the Northern Territory, and Departments of Agriculture and Conservation and Environment in Victoria are some examples of organisations with people working in this area.
This work was highlighted at a direct seeding conference in Adelaide in May 1990.
Techniques for direct seeding
As seed is often very small, it is generally mixed with a carrier to help achieve desirable sowing rates. Suitable carriers may be:
• bran or pollard
• dry sand
• or, for fluid drilling, laponite inorganic colloid.
Seed plus carrier can be sown into the site by:
(a) spreading it onto cultivated ground using:
• hand sowing
• super spreader (hand operated or tractor driven)
• small seeds box on combine
• sod seeder;
(b) sowing with a purpose built machine such as the Roden MK III or the Weatherly tree seeder. These machines can:
• spray weeds with knockdown and pre-emergent herbicides
• scalp a trench and sow seed into it
• cover seed with soil using finger tyres and a presswheel
• cover seed with bitumen emulsion to enhance germination;
(c) a belt drive or vacuum operated precision seeder for sowing some species singly. However, these are generally unsatisfactory for the very small seeds;
(d) a fluid drill, which is a device for sowing (usually) pre-germinated seed in a gel carrier (laponite inorganic colloid). This can be used in conjunction with carbon banding using a layer of activated carbon over the gel, and then overspraying with pre-emergent herbicide.
Correct site problems:
• Acidity - work in the required amount of lime.
• Waterlogging - construct drains or hills.
• Salt - construct drains or hills and leach salt from soil profile to be planted.
• Hardpans - rip to break up hardpans and aerate the soil.
• Red-legged earthmite, thrips, aphids, and slugs can devastate very young seedlings.
• Rabbits and hares prune seedlings part way up the stem, destroying plant form.
• Birds such as cockatoos, magpies and galahs can moderately to severely damage seedlings - destroying form or causing mortality.
• Crickets can ringback seedlings just above ground level.
Strong weed competition can drastically reduce seedling survival, especially in their earlier stages of growth. Vigorous spring weeds seem to be the most damaging, whereas some summer weeds such as wireweed do not seem to have much effect as tree seedlings are better established.
Monocotyledons can be controlled with selective herbicides but, in doing so, care must be taken to ensure that the bare ground thus created is not swamped with dicotyledon summer weeds.
This is mostly a question of correctly timing sprays.
1. Time of sowing is important. Early trial results suggest that late August to early October are the best times, with October plantings being better in a year with a late spring.
Table 1. NAP Direct seeding trial results
2. Detailed research sites have shown large differences in seedling establishment due to factors such as aspect, soil type, drainage available moisture and exposure.
Treatment differences showed that on heavier soils herbicides were more effective than cultivation and most sites showed a good response to use of bitumen post planting.
Table 2. Effect of site characteristics on seedling establishment
Direct seeding of trees and shrubs can be very successful.
The most important factors determining success are:
• the site must be well prepared;
• the correct species for the area are sown at the best time;
• young seedlings must be protected from pests and weeds.