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10 gardening questions you always wanted to ask


What's the difference between compost and soil?

Soil is natural, produced as weather and natural organisms break down rocks, plant debris and decaying animal matter. It varies from area to area depending on the underlying rock and whether it has been moved around, like on a new housing estate. Compost comes in two types: the first is made by accumulating plant debris from the garden and letting it rot down; the second is made and bagged by manufacturers, using materials like peat or coconut fibre (coir). Home-made compost is ideal to use as a soil improver, but it is not sterile and its make-up depends on what went into it. Bought, bagged compost should be clean and consistent so its better for sowing seed, indoor plants and growing plants in containers.

John Innes compost is based on sterile soil and is ideal for growing different types of plants. It comes as Seed & Potting compost, but also at three different strengths according to how much fertiliser has been added to it. Number 1 is the weakest and is good for bedding plants, alpines and smaller plants; no.2 is stronger and is suited to medium-term plantings like shrubs; no.3 is the strongest and is suitable for long-term plantings of trees, conifers and bigger shrubs. Most plants work their way up to needing no.3 and, if you put a small plant into it straight away, the high fertiliser level may scorch the roots.

How can I tell if Iím buying a good plant?

Whenever you buy a new plant, you need to check certain things:

  • are there any signs of damage to the leaves or stem?

  • can you see any insects or have the leaves got holes in?

  • are the leaves a good colour (pale or yellow colours can indicate it is sickly or under-fed)

  • the roots (yes, have a look) should be plump and healthy-looking

  • the compost in the pot should look fresh and weed-free

  • no (or few) roots should be poking out of the pot base

  • is the plant the right size for the pot?

A plant that is discoloured, deformed or eaten is best avoided. Likewise, any plant that is 60cm tall in a 9cm pot is not going to perform well, because it has been forced into growth to make the sale.

If you buy from a good nursery or garden centre, they will offer a guarantee so you can take it back for replacement if it dies.

How can I tell if Iím buying the right plant?

Just because you like a plant, donít assume itís the right one for the gap youíre intending to fill. Some like a hot, dry spot whereas others prefer moist soil and part-shade. Some will curl up and die in a windy position whereas others can take it. A bit of research will tell you which is which and consulting a good book or a reputable growerís website will save you wasting time, effort and money.

There are a few easy tips here, though, such as plants for a hot, sunny area often have grey, silvery and/or hairy leaves. This is because the plants have adapted over the millennia to survive the heat by reflecting it back and having the hairs to keep moisture trapped as a coolant. Plants for shady places often have dark green leaves to make the most of whatever light is available.

You need to know the pH of your soil too, because some plants need acid soil while others need alkaline. This is easy to do using a small kit from the garden centre. You can channel your inner scientist by taking a soil sample from about 4cm down in the garden and testing it with the kit.

How do I plant a plant?

The obvious answer to this is ďgreen side upĒ, but if youíre planting bulbs or corms, this is actually a serious question because sometimes you just canít tell. A plant with roots needs to go into a hole in the ground that is about double the size of the pot it came in and a tiny bit deeper. This means that as the new roots begin to grow, the first soil they reach is loose enough to grow into, rather than being hard (in which case, they simply circle the original hole until they die). A thin layer of soil over the original pot surface reduces evaporation and lets the plant establish more quickly. A tall plant may need to have stake to keep it supported, so push this into the hole as you plant. It needs to be as close to the stem as you can get it without damaging the roots. Make sure the wind will blow the plant onto the stake rather than away from it (it may break) and always tie the plant to the stake, not the other way round or there wonít be room for the stem to expand and the buckle may damage the stem.

If youíre planting into another pot, choose one that is about one size bigger than the one itís in. If you jump up too much, the sheer volume of wet compost in a big pot will rot the small new roots.

If you do have bulbs or corms to plant and you really donít know which way up to put them, simply pop them on their sides. As they begin to grow, the roots will pull the plant the right way up thanks to good old gravity.

Whatís important about soil?

Soil is all too often dismissed by the uninitiated as ďdirtĒ, the stuff that happens to be on the ground and is useful for holding plants upright. Itís actually a massively complicated medium consisting of billions of organisms that help the world to function and without good soil, weíd be in trouble. Think of it as an ice cream sundae, with a base layer (rock), a middle layer (sub-soil) and a topping (top soil). Sub-soil tends to be a bit airless and isnít high in food for plants, but the top soil is the good, fertile bit where the plants will thrive. Worms move around constantly in good soil, improving the food levels as they digest decaying material and keeping it aerated. A regular supply of leaves and other plant material (including mulches) on the soil surface provides the worms and all the other organisms that dwell in the soil with food and they, in turn, provide the food that keeps the plant healthy. Circle of life! If you dig into the soil, you risk mixing the top soil with the sub-soil and this will reduce the overall fertility, so try to keep the two separate if you can. There is usually a colour difference, with the sub-soil looking darker, even bluish due to lack of oxygen if the soil is prone to waterlogging.

Why do I have to worry about watering?

Although we may be happy to have a long, dry spell of weather, itís potentially damaging to plants. Itís actually very easy to forget just how long itís been since rain fell if youíre enjoying leaving the umbrella at home. Plants need water all the time, because all their functions and processes depend on it. They extract nutrients from the soil, but they donít have teeth to chew with so they rely on the food being dissolved in water that their roots can absorb. Any plant in a pot is entirely dependent on you for its food and water. If you donít provide - or at least top up - what it has, it will begin to suffer. How much water a plant needs depends on lots of things, like air temperature, size of plant, where itís growing and what it is. A plant like Clematis, which is related to buttercups that naturally inhabit damp areas, needs damp soil around the roots at all times or it will get mildew on the leaves, hence the old saying that they prefer ďtheir heads in the sun and their feet in the shadeĒ. If you want your plants to grow well, take the time to check them regularly. Youíll soon find out which ones are thirsty and which can last longer between waterings.

Whatís pH & why does it matter?

The pH scale measures acidity and alkalinity, on a scale from 1 to 10. In gardening terms, we only really care about the middle section from about 4 to around 8, because soils donít often go above or below these numbers. A figure of 6.5 is classed as neutral and offers the most scope for planting. Once you go below pH 5.5, you are dealing with plants that will struggle in anything except an acid soil. This includes the Ericaceous group of plants like Rhododendrons, Camellias, azaleas, some heathers and Pieris. Above pH 7 you need the plants that prefer alkaline, or chalky, conditions like Dianthus and many lavenders. So, the pH matters when you are choosing plants because youíll waste time, effort and money if you get the wrong ones. There are always pockets of soil in an area that donít conform with the rest of the area, but just because someone is growing beautiful Rhododendrons down the road doesnít mean theyíll grow in your garden, so get a little pH kit from the garden centre and test your soil. Ideally, for an overall picture, take 4 or 5 samples from a depth of about 5cm from different parts of the garden, mix them together and take the test sample from the mixture. If you only want to test a single area, then just take one sample.

Why does it matter which way the garden faces?

In the UK, most of our weather blows in from the south-west, although the coldest winds come from the north. Thereís no direct sunlight from the north, but if your garden faces south, itís likely to be hot and sunny, especially in the middle of the day in summer. The hardest part of a garden to plant is one that gets cold wind and little sun. You can do something about exposure to winds by planting a hedge or putting up a fence for shelter, but a lack of light is more difficult to deal with. You need to try and match the plant to the place as closely as possible. If you have a sunny, south-facing wall, grow fruit against it (grapes, peaches or nectarines) or a tropical climber that prefers a hot spot (Trachelospermum or jasmine). A north wall is better suited to an evergreen clematis (C. cirrhosa), climbing hydrangea (H. petiolaris) or a tough wall shrub like Pyracantha.

Is fertiliser important?

If you were standing in one place for years and no-one fed you, how good would you look? Out in their natural habitat, plants achieve a balance where the leaves they shed are recycled by organisms in the soil to provide nutrients to feed the plant the following year. In a garden, we tend to have a much greater density of plants together, all with different food requirements. Some need a lot, some not so much. Some root deeply into the soil, others are shallow-rooted. Competition for food is intense and once itís gone, it takes time to replenish which is why we apply fertiliser. A plant in a pot is entirely dependent on you to keep it fed and watered, whether this is a tomato in a grow-bag or a shrub in a big permanent container. Chances are, you would feed the tomato, but forget the shrub because its always there, but the shrub actually needs the food every bit as much, just not as often. The fertiliser in compost is used up quite quickly and after that, you need to apply food at least once a year to keep the plant healthy.

What's pruning and why do I have to do it?

Pruning is the act of trimming a plant to keep it healthy or control the way it grows. Unless itís a fruiting plant that needs regular pruning to keep it cropping, simply follow the 4Ds rule and remove any Dead, Dying, Damaged or Diseased material. If you donít know when to do it, then the answer is immediately. If you want to prune to control the size or shape of a plant and arenít sure when to do it, then do it immediately after it flowers so it has the maximum time to grow and set flower buds for next year. Most evergreen plants have two growth spurts a year, in spring and autumn so prune them just before one of these and cut slightly further back than you want so the new growth covers the cuts and you end up with it the right size. Pruning fruit is more complicated and you really need to consult a good quality book.

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This story was published on: 10/10/2019

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