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Gardening with Containers

"Gardens... should be like lovely, well-shaped girls: all curves, secret corners, unexpected deviations, seductive surprises and then still more curves." - H.E. Bates

A well placed container adds a highlight as well as a planting opportunity in any garden - big or small, providing punctuation to halt the eye as it scans the view to give structure to the design. Where space is restricted or on a bland expanse of paving a tastefully planted container brings life and contrast. More tender plants can be grown as the planter can be moved under cover in the winter or bulbs and annual plants can be added to areas of perennial plants when they are out of bloom.
Anything the right shape can be used as a planter, so depending on the style of the garden the choice is limitless. In a formal setting it is best to stick to classical shapes in metal, stone, terracotta or an artificial material which resembles these. A cottage style garden is more forgiving and recycled objects can be used, eg. old buckets, sinks, wheelbarrows, or the old standard a half-barrel, even Wellington boots have been used. But don't get carried away, a mass of unrelated objects looks a mess, it is best to choose a theme, eg. a group of old metal watering-cans and buckets. The important thing when container gardening is to ensure it holds enough planting medium and has drainage holes. (If you choose this style you can claim to be following the Arte Povera movement which was formed in the 1960s by a group of artists who used found objects to create their art - the translation from Italian is "poor art".)

The recent introduction of fibreclay containers give the more authentic look of terracotta or lead, the lighter weight is easier for handling and where weight is a factor on a balcony or a rooftop. It would appear from recent observation that these pots may not be long-lasting as the clay-like coating seems to decay, exposing the fibrous mesh and the pot collapses. Plastic pots do not weather to any extent and although they are lightweight they always look artificial.
Terracotta can be problematic where it is exposed to frost which causes it to shatter. It absorbs moisture and this varies depending on the firing temperature. Lower fired pots are softer and will absorb more so the extra expense of harder, high-fired clay may be worth it in the long run. As a precaution the container can be treated with a water repellent. The large pot in the picture below has spent many years outdoors throughout winter. Before it was placed there it was painted with a couple of coats of a clear, silicone-based product which is normally used to prevent water penetration into brickwork.


The shape of this urn is unsuitable for planting, but
it has enough character to work on its own.

The choice of container can have a great bearing on the result. A plain one would probably be best for a display of flowering plants, otherwise the two elements will clash. - something like putting floral wallpaper with a floral sofa on a heavily patterned carpet. A more decorative container looks better with a single species of a foliage plant, eg. a hosta or a grass, or to be left unplanted and used sculpturally.
Size also matters. There is little point in using a container less than 30cm in diameter as it cannot hold enough planting medium to retain sufficient moisture for any length of time. The walls also need to be thick enough to keep the roots cool in hot weather and from freezing in the cold. Terracotta pots have been found to be best at maintaining a cooler root-ball in hot weather. In prolonged cold spells the whole container may have to be lagged to prevent the rootball from being damaged as it is more exposed than it would be in the ground. For containers with thin walls such as galvanised steel, the roots can become too hot in the summer or frozen in the winter. A lining of insulation such as ploystyrene foam or heavy duty bubble-wrap can be placed against the inner walls prior to filling with planting medium. Terracotta pots lose moisture through the sides so an inner lining of plastic will reduce evaporation by this route, glazed ones are more suitable - if a little less attractive.

Containers which have a narrower rim than the full width are less suitable for more permanent planting as it makes re-potting very difficult. To remove a plant from such a pot, cut down through the rootball inside the rim to remove the bulbous portion of the roots with an old kitchen knife, or a spade in larger containers - this may require a hammer to drive it down like a chisel, hitting the treads.
It may seem obvious, but a large container should be placed in situ before filling as it may be difficult to move when planted up. Also if the chosen plant is in a much smaller pot it should not be placed directly into a large container as this will affect its development. It would be better to pot it on in progressively larger pots to allow the root-ball to extend slowly outwards; placed immediately into a large pot the roots grow to the sides leaving a root-free zone in the middle. The progression of pots can be inserted in the large decorative container surrounded by compost.

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Growing plants in a container is much different to growing them in the open ground. The roots are restricted and the growing medium has to provide everything the plant needs, it cannot seek them out as it would naturally. For long-term planting a soil based medium is best as it provides stability for the roots, holds nutrients longer and is easier to re-wet if it should dry out. Multipurpose compost is more suited to short-term planting, its nutrients are quickly used up and if it dries out it is difficult to re-wet.
One of the saddest sights is the tree or shrub with a few yellowing leaves sitting in a container stuffed to the brim with ancient multipurpose compost which has a covering of moss. It may be possible to rescue it by extracting the root-ball, scraping away as much of the old compost as possible and any matted roots. Then repot with a loam-based compost, a little extra feed and plenty of water.

Whatever growing medium is used it should drain easily to avoid waterlogging, but have enough moisture-retaining material to keep the rootball from drying out. A loam-based medium made to John Innes Number 3 formula has the right balance for long-term planting. A mixture of good topsoil, sharp sand and multipurpose compost with some additional long-acting fertilizer, is a good alternative and is much cheaper than commercially produced John Innes composts - a mulch of gravel or bark should keep weeds to a minimum. Use two large buckets to mix the ingredients, passing the mixture back-and-forth a number of times until the constituents are evenly distributed.
Superabsorbent polymer granules made from a special plastic called polyacrylate, bond with water molecules to hold many times their own volume to increase the moisture retention of a growing medium - they are particularly useful in hanging baskets. They are a development from the nappy (diaper) industry where they are used to keep baby dry, whereas gardeners use them to keep their babies wet! The polymers used for nappies last in the soil for about 5 months, but horticultural grade polymers should last for 4 to 5 years so it may be more economical for short term use to disect a nappy and extract the beads - unused of course, although a wet nappy would have additional nutrients!

Another drawback is that the plants are more vulnerable to pest damage. The Vine weevil is a particular problem and it is a good idea to check for the grubs in the autumn. Succulent rooted plants are their favourites, eg. Primula, Sedum and Echeveria; if they are showing signs if dieback or wilting the grubs may be the cause. It is a good precaution in the late autumn to remove the compost and wash the roots to clean off any eggs, before re-potting in fresh growing medium. A drench of *imidacloprid insecticide is the chemical treatment.

Raising the container off the ground is necessary to allow excess moisture to flow away, when it is in contact with the ground the moisture becomes trapped. This can rot the roots near to the base and in frosty conditions a terracotta pot which is saturated may shatter. Three 'feet' are ideal, as they can cope with uneven surfaces better. There are decorative ones, but small pieces of terracotta can be placed out of sight underneath the pot. The drainage holes need to be protected on the inside to prevent the planting medium from blocking them. Broken terracotta has been the traditional material, but anything which will not rot away will do; broken up polystyrene foam packaging such as plant trays are commonly used and have the advantage of being light. However for a tall plant like a tree some bricks or coarse gravel may be more suitable to give more stability to the container.

Do not fill the container to the brim as this will make watering difficult. Leave a least 3cm between the surface of the compost and the rim. This space can be filled with water and it will percolate down around the roots. If the compost becomes dry it shrinks away from the sides and the water escapes down the void. Break up the surface to block the space or if possible block the drainage holes temporarily to hold the water long enough for it to be taken up by the rootball. A smaller container can be immersed in a large bucket of water for an hour or so, until the air bubbles stop - it may have to be weighted down at first and the addition of two drops of detergent to a bucketful of water aids the rewetting process.
If sited near to the house, a wall or a hedge, the watering requirements can be affected as even during wet weather it may be in a rain shadow under the eaves, or on the lee side so little rain reaches the pot. The roots of a plant reach beyond its canopy when it grows in open ground, but in a pot they cannot, and the leaves can shed most rain water outside the pot.

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A large decorative container need not be planted up directly, a series of large pots which fit inside it can have different plants which perform at different times of the year so that there is always something of interest on show. They can be kept in a 'nursery area' when not on display. The spring show could be of bulbs such as crocus, hyacinth and daffodils. When these begin to fade the pot can be replaced with one of tulips. The next in the series can have annuals. Lilium candidum, the Madonna Lily, has wonderful white trumpet flowers with a wonderful scent and would make a good choice for the late summer display.
One pot could have a more permanent display of foliage plants, to fill any periods when there are none in flower. This also suits tender plants which can be taken under cover for the winter while a frost-hardy display replaces it.

Some plants are better suited to container growing than others. Those which naturally grow in clumps tend to do better. Perennial plants which spread out, grow to the sides leaving a dead patch in the middle. Agapanthus is a good candidate and with fleshy roots it is very tolerant of dry conditions - also it tends to bloom better with restricted roots. If a plant is not suited to the soil conditions in the garden a container can be used to overcome this, eg. using an ericaceous compost to grow an acid-loving plant in a garden with chalky or alkaline soil .

To have some vegetables in a small garden or to get started on the pleasures of growing them, a container is ideal. A sowing of mixed salad leaves makes an easy first step. It is best to have a large pot at least 45cm across and the same deep to hold enough growing medium. Carrots do quite well and potatoes can be grown, but need to be planted near the bottom then more compost is added as the shoots grow. Others to try are 'Gardener's Delight' or 'Tumbler' tomatoes, raddish and peppers. The main thing to remember is that they need lots of water and feeding to produce a bumper crop. A container of herbs near to the back door is handy for nipping out while cooking.

A small tree in a container will need the compost to be replenished every few years. This can be done in the dormant season. Remove the rootball and tease away about 3 to 4cm of the outer roots and compost - so choose a container which is widest at the top. Repot it with fresh growing medium and water well. This is similar to the treatment of a bonsai tree which has to have its roots pruned regularly to keep it 'growing' well. A topiarised box plant is ideal for a container, but it requires regular watering and feeding to keep it performing well, unfertilised the foliage becomes yellow and sparse.

Feeding the plants in the container is important, particularly if a multipurpose compost has been used - the fertilizer in these mixes only lasts for a few months so needs to be replenished. As a general long-lasting feed the coated pellets such as Osmocote contain roughly equal proportions of the three major nutrients - Nitrogen (N), Phosphate (P) and Potassium (K). For a flower display use a feed higher in Potassium (sometimes called Potash). The organic alternate is bonemeal. The use of too much fertilizer can cause a build-up of harmful salts which reverse the osmotic flow and the roots are unable to take up water.


This replica amphora adds interest even in winter
when the perennials are 'resting'

There is a vast range of containers available and they can be used in a number of ways around the garden. Also the number of plants which can be grown in them is large as well, from annuals, perennials, shrubs and trees to vegetables and salads. The container can be used unplanted as a decotative object in its own right, or even sealed and made into a water feature. So whatever the situation there is no excuse for not having one, or preferably more, in the garden. But do not get carried away, too many pots can look untidy, try to limit the number to one, or a neat group per vista. "Less is more" is a good rule to follow, the term used by Mies van der Rohe credited as a pioneer in modern architecture.



*Imidacloprid is a neonicotinoid compound and these have been suggested as as causative agents of colony collapse disorder (CCD) in Honeybees.


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