What is the difference between a 'compost' and a 'growing medium'?
They are often used for the same purpose - to describe a material used to grow plants in containers. Technically, however, a ‘compost' is the product of a composting process (e.g. what is produced in a compost bin) so a peat or bark based growing medium is not really a compost even though they might still be given that name!
What is ‘Multi-purpose Compost' made of?
Most Multi-purpose Composts are made from a blend of peat with other materials such as bark or green compost. They also contain lime and some fertiliser so that the pH and nutrient levels are suitable for a wide range of plants.
Why do you need to use a special growing medium for seeds and cuttings?
Seedlings and cuttings are sensitive to high nutrient levels as they have not yet developed proper root systems; therefore they are raised in special low-nutrient media. For small seeds a finer grade mix with fewer large particles may also be needed to create the right conditions for germination.
Does it matter what sort of bark is used in growing media?
Yes, the bark of deciduous trees contains toxins so bark from coniferous trees such as pine and spruce is used. This still has to be matured or composted before use however.
How quickly do I need to use up a bag of growing media?
It is best to use a bag within 3 months of purchase or at least by the end of the season it was purchased in, however peat-based media which are kept in a closed up bag in a cool place will keep for up to 12 months. If you are not sure about older bags use them for less demanding purposes such as filling up tubs and planters rather than pricking out young plants.
Is it OK to re-use grow-bags?
It is not recommended as there could be disease in the bags from the previous crop and the nutrient content and structure of the growing medium area likely to be poor after being used for a cropping season. It is best to empty used grow-bags on to your flower-beds and dig in to improve the organic matter level in the soil.
Why can't I just use my garden soil in pots to grow plants?
Because you are growing a plant in a much smaller volume of medium than it would explore if growing in the soil and therefore a material with a higher air and available water holding capacity is needed. If you use soil alone it will tend to bake hard in the summer and become waterlogged in the winter so it will be very difficult to manage.
What is pH?
pH is a measure of how acid or alkaline something is. Chalky soils are alkaline and have a higher pH (7-8). Sandy or peaty soils are naturally acidic and have a lower pH (4-5). Growing media are formulated with a pH to suit a range of plants - usually around 5.5 - 6.5, except for special mixes for acid loving plants such as Rhododendrons or Azaleas which are adjusted to around pH 5.
Why is it best to use rain-water to water plants like Azaleas in pots?
Because they are acid-loving plants and in many parts of the UK the tap water contains chalk and will gradually raise the pH of the growing medium in the pots causing these types of plant to show leaf yellowing due to lock up of iron at high pH.
What is vermiculite?
Vermiculite is a mineral that has been heat-treated to expand the layers in it into a lattice-like structure therefore it can hold a lot of air and water. It is used in specialist mixes sometimes and can also be used to cover seeds.
Why is the Government trying to reduce our use of peat?
Because peat formation is very very slow, it is not considered sustainable to harvest it at the current rate. Once peat-bogs are drained the plants and animals associated with this type of ecosystem are lost and is difficult to restore them afterwards to exactly the same condition as they were originally. Peat bogs are also a carbon sink in a similar way to rain forests. Much of our peatlands have already been drained for agriculture, forestry and building; it is not just extraction for horticultural use that has caused the problem.
Are peat alternatives as good?
Yes, they can be; they just need different management sometimes (e.g. watering and feeding). Reduced peat products made from a blend of peat with other materials often out-perform 100% peat mixes in trials. Some species are difficult to grow in completely peat-free media, for example carnivorous plants and some ericaceous species.
Do I need to treat peat-alternatives differently to peat?
Peat-free mixes can be more free-draining than peat mixes and so may need more little and often watering. Peat-free media based on bark type materials may need a bit more feed to compensate for ‘lock-up' of nitrogen by the bark.
What are GMA members doing to reduce their use of peat?
GMA members are using non-peat materials blended with peat in many ‘multi-purpose' growing media and ‘Grow-bags' and also in growing media for professional nurseries. They are continuing with trials with reduced peat and peat-free media to try to find ways of reducing peat use even more.
Care has to be taken however to ensure that alternative materials are safe to handle, economically viable, available over the long-term and that they have better environmental credentials than peat (not worse!).
How do the watering requirements of reduced peat and peat free growing media differ from those of peat media?
It depends on the type of materials used in the mix but many non-peat materials such as bark, woodfibre and coir are more free-draining than peat and may therefore need more 'little and often' watering than peat, which has a higher water holding capacity and is more tolerant of longer intervals between waterings. If they dry out, however, these materials are often not as difficult to re-wet as peat. They may appear dry on the surface but still be moist underneath ( unlike peat for which surface dryness is a good guide to overall moisture content) so care is needed to avoid over-watering.
The reduced peat mix I am using appears to have a white mould growing on the woody material, is this harmful to plants?
This is probably just a saprophytic fungus and it will not cause any problem but if you are worried contact the manufacturer.
What sort of growing medium is best for a gardener to use for sowing seeds?
For good seed germination you need a growing medium that is fairly fine textured (so it holds sufficient moisture and allows good seedling emergence and root development). The growing medium should also have low nutrient levels because young seedlings will be damaged by high salt levels. Multi-purpose growing media products can be used for seed sowing but may be too coarse textured for fine seeds and you will usually get better results using a specialist seed sowing product. For larger seeds the texture is less critical than with small seeds but multi-purpose products with a high bark/woody material content will not give such good results as a finer- textured product.
Why does my bag of multipurpose compost have pieces of woody material in it and look different to compost that I have bought in recent years?
All GMA members are striving to reduce the amount of peat in their products for environmental reasons so materials such as bark, green compost and wood fibre are being blended with the peat - this should not affect the performance of the compost.