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The first records of seaweed being processed come from China in 2700 BC. The Chinese and Japanese have been using it for both human and animal consumption ever since. Initially U.K. agricultural use was limited to the use of bulk fresh seaweed as a manure and soil conditioner. Today the use of various processed seaweed products has made life much more convenient. The three main types of product are -

Dried seaweed.
Calcified seaweed.
Liquid seaweed extracts.

Opinions differ widely on the merits of seaweed in horticulture. To some they are a panacea for all ills and to others just vague alchemy. As always the truth lies somewhere between these extremes. So what does the scientific research tell us?
Seaweed extracts act as plant growth stimulants: their effectiveness may be influenced by the species included and the manufacturing process used. Overall plant performance can be shown to improve due to their use and affect several areas including -

Plant growth.
Protein and carbohydrate production.
Prolonged chlorophyll production and increased levels of photosynthesis.
Beneficial results are most noticeable when plants are under stress.

These positive results can be attributed to various active ingredients found in seaweed products. More than sixty elements have been identified including macro nutrients and trace elements. Whilst these can go some way to improving plant nutrition and well being they do not tell the whole story. Seaweed extracts alone are unlikely to remedy a severe nutrient deficiency which should be addressed separately.
Seaweeds contain concentrations of auxins, gibberellins, cytokinins and betaines which are all types of hormones that can stimulate growth. In particular cytokinins are closely involved in cell division, protein, carbohydrate and chlorophyll synthesis. Betaines are modified amino acids that can act as anti-stressors in both biotic and abiotic stress conditions. Glycine betaine has been shown to act as an osmoprotectant, to enhance water utilization and to have a role in frost resistance. Ultimately and because of the complex makeup of seaweed extracts it is very difficult to put a finger on a single element that is categorically responsible for the beneficial effects of the products. However, what can be shown is that great benefit can be derived from the use of seaweed on plants of all types and the effects can be very rewarding.
Scientific trials have confirmed measurable results across a range of aspects of commercial plant production including -

Higher marketable yields - Potatoes showed improvements up to +36% by weight from U.K and U.S trials following 2 foliar applications of liquid seaweed extract. Similarly carrots show up to +23% increase, tomatoes +31%, onions +17-50%, peppers +12%, pears +45%. peaches +13% and so on across a huge range of test crops.

Better quality and improved shelf life - Tests showed improved formation, skin colour, texture, sugar content and juice purity. This was attributed to increased leaf area and extended chlorophyll life leading to increased carbohydrate production and delayed senescence.

Increased resistance to pest and disease - Research suggests that constituents of seaweed extracts, in particular the betaines, induce systemic acquired resistance (SAR) to various stress situations including pest attack. Betaines, at the concentration present in seaweed extracts, have also been proven to reduce soil nematode invasion and inhibit powdery mildew, stem rust and botrytis. Extracts have also been proven to considerably improve the effectiveness of pesticides when applied in unison.

Increased nutrient uptake - Practical evidence in a series of U.K trials indicated that plants made better use of available nutrients when treated with seaweed extracts to the extent that fertiliser use could be reduced without affecting overall yield.

Stronger growth - One of the benefits of using seaweed is that it stimulates strong healthy growth, not the rapid soft growth associated with high nitrogen applications. Vigorous root systems are particularly noticeable thus giving the plant better access to nutrient and water supplies.

Increased frost resistance - Seaweed extracts contain both amino and modified amino acids and it is thought that these assist the plant to withstand cold.

In summary it is evident that seaweed products produce their best results where optimum conditions are lacking or where, for any number of reasons, a plant is under a degree of stress. As far as the bonsai enthusiast is concerned growing trees in very limited volume of soil will always place a degree of stress upon the plant and anything that can be done to reduce this will be of benefit long term. Using seaweed products alongside a good fertiliser regime can only improve our results and lead to healthier more beautiful bonsai.