Monthly Archives: June 2019

How hardy plants are

Most gardeners are aware of zone systems that help us determine the plants we can grow in our gardens. Perhaps the most commonly used zone map in North England is the UK Plant Hardiness Zone Map, which is based on area temperature readings. It defines 11 zones, with each zone determined by the average lowest temperature in that area. Zones rise in increments of 10 degrees: the higher the zone number, the warmer the area. However, these zones are only guidelines, not hard and fast rules for what you can and cannot grow. Here are some additional factors that affect your choice of plants:

UK Plant Hardiness Zone Map

UK Plant Hardiness Zone Map

The UK Plant Hardiness Zone Map divides areas by temperature, but it is not perfect. Each area and garden has mini-zones (Water Retaining) that no zone map can show. Microclimates are areas that are different from the surrounding area in topography, exposure to wind and sun, drainage, or other factors. The differences in these microclimates may add or subtract a zone or even two. For example, a south-facing wall may be a zone warmer than the rest of your yard because the heat absorbed by the wall warms the soil in that particular area.


The weather also plays an important role because zones represent the average low temperature for an area. A particularly harsh winter in Zone 5, for instance, might actually be a Zone 4 or Zone 3 winter. Likewise, an especially warm winter might only be as cold as a Zone 6 or Zone 7 winter. This is especially true for gardeners who live on or near a border between two zones.

Plant type

The kind of plant you grow makes a difference. Trees, shrubs, and woody vines are exposed to whatever winter temperatures your area receives. Herbaceous perennials and biennials, on the other hand, die to the ground each year. This keeps their crowns protected in the soil from some of the harshest weather. The fact that they die back also allows you to mulch them. Several inches of winter mulch can help moderate the effects of cold weather.

Plant health

Plant Health

Plant Health

Strong, healthy plants withstand harsher conditions than plants struggling to survive. If your plants were stressed by a summer drought, they might be more susceptible to winter damage than if the summer had been moist and mild, for example.

Soil type

Soil Type

Soil Type

Soil also plays a role in plant hardiness. Many herbaceous plants survive better in sandy soils because these soils drain well. Clay soils, on the other hand, retain water. When the water freezes around plant roots, it can compromise their hardiness.


The UK Plant Hardiness Zone Map does not take into account summer heat, either. Many plants, such as Himalayan blue poppies (Meconopsis spp.), do not withstand warm temperatures, even though they tolerate quite a bit of cold. The American Horticultural Society has created a Plant Heat-Zone Map that marks areas in the U.K. by the number of annual days above 86°F (the point at which plants suffer heat stress). Heat-zone ratings are gradually appearing in catalogs, books, and plant labels.

Other zone systems

Natural Resources UK and Agriculture and Agri-Food UK released a map of UK zones that is quite different from the UK map. Additionally, many gardeners on the West Coast rely on a zone system created by Sunset Publishing that takes into account other factors besides temperature. The important thing to remember is that no zone system is perfect, and should only be used as a guide.

What type of soil should I use

Long ago, a gardener’s choices of soils and amendments were simple—dirt simple, in fact. Maybe you’d enrich your soil with compost or animal manure or break it up with sand or gravel, but the options were pretty basic.

These days, most well-equipped garden centres have an enormous wall or two devoted to packaged soils. Many are things our great-grandparents never heard of—like biosolids, polymer crystals, and soilless growing medium. Some boggle the imagination: Bags of garden soil? Isn’t that what’s in your garden already? To help you navigate the confusing world of soil amendments, here are some of the most common questions—and tips that will help you get the most from your next trip to the garden centre.

What’s the difference between a soil, a soil amendment, and a soil conditioner?

Soil For Plants

Soil For Plants

Soil is dirt. A soil amendment is anything you add to the soil to improve it. And so is a soil conditioner, except that savvy markers have created a variety of blends of soil amendments and slapped the label “soil conditioners” on them.

Why would you grow plants in a soilless mix?

Soil weighs down mixes. A soilless mix is lightweight and is easy to sterilise, perfect for seeds and trickier container plants.

What’s the difference between a mulch and a soil amendment?



To use something as a soil amendment, you need to work it at least several inches into the soil. Mulches, on the other hand, are spread on top of the soil to conserve moisture and suppress weeds.

Confusion occurs because some materials can be used as either an amendment or a mulch. Compost and finely chopped or well-rotted leaves, for example, can be worked into the soil or spread on top of it for mulch. Bark dust, too, can be used as either a mulch or a soil amendment (but don’t mistake the finely ground substance for bark nuggets or materials labeled simply as bark mulch). You shouldn’t work sawdust or any fresh organic material directly into the soil because in the process of breaking down, it robs nitrogen from soil. However, it’s fine to add such materials to the compost pile, where they’ll break down before you use them.

How to get your money’s worth

If this information makes your head spin, it’s not surprising. Creative marketing teams give all sorts of names to all sorts of soils and soil-like substances in bags. Actual content can vary according to company. When in doubt, read the label and talk to a knowledgeable sales assistant or manager. When shopping for bags of soil, potting mix, conditioners, and amendments, keep in mind the following:

Focus on details. Label-ing laws for garden products vary by state, but the more specific a manufacturer is about what’s inside, the more likely it is the bag contains something worth boasting about. The reverse is also true. If there’s very little information about what’s inside, it’s probably not worth crowing about.

Buy volume, not weight. When it comes to potting soils, mixes, and conditioners, weight has almost nothing to do with quality. The weight could be coming from dark, heavy, low-quality peat, water, or even just plain sand. Heaviness, in fact, is more often a sign of poor quality than of good value. As a rule, go for larger bags that are lighter.

Look for key words. Buy quality. Look for words such as “professional” or “premium.” These words aren’t a foolproof assurance of quality, but they’re a good sign.

Don’t think that mix-ins are a must.

Water Retaining

Water Retaining

Potting mixes often include fertilisers and special moisture-retaining additives. The fertilisers added to potting soils are usually short-term, so keep in mind that you’ll still need to fertilise. Also, hydrogels or polymer crystals (also called water-retaining crystals) claim to help potting mixes retain moisture so you don’t have to water as often, but there’s scant evidence that they’re truly effective. When in doubt, don’t pay extra for these mix-ins.

Feel it. Bags often break open, so if you see a ripped bag of the brand you’re thinking about buying, put some of the mix in your hand and feel it. Or ask a clerk if there’s any already open that you can feel. If you’re planning on buying several bags, ask the clerk to open a bag so you can feel it.

Stick with your favourites. Once you find a product and a brand you like, stick with it. Many a gardener has opened up a bag of garden soil or topsoil to find something disappointing. Save yourself the surprise and expense by staying with what you like. As a bonus, you’ll find yourself spending a lot less time in front of that array of soil-like products, trying to figure out what’s in the bag.

The Little Bags

In smaller bags, you’ll find the following soil amendments, some of which double as fertiliser or can alter the soil’s pH. As their smaller size suggests, they’re added to soil in smaller proportions. Read labels carefully.

Bonemeal: Made of finely ground, steamed animal bones. This phosphorous-rich material is a popular fertiliser for bulbs, but specialty bulb fertilizers are more effective. Slightly improves soil texture. Work into planting holes, or sprinkle on the soil surface and rake in. May attract animals.

Bloodmeal: Made of dried animal blood. A rich organic source of nitrogen. Slightly improves soil texture. Sprinkle lightly on the soil surface or follow package directions and rake in. May attract animals.

Gypsum: Breaks up some clay soils, but its effectiveness is limited. Spread on the soil surface and then dig in to a depth of several inches, following package directions to get the right amount.

Lime: Made of ground limestone. Raises soil pH. Should be applied only on acid soils to accommodate plants that need more alkaline conditions; don’t automatically apply every spring. Processed lime takes effect in a few weeks; dolomite takes a few months.




: Lowers the pH of the soil, making neutral and alkaline soils better for acid-loving plants such as azaleas and hydrangeas. As with lime, use only if you know your soil’s pH.

Vermiculite: Bits of the mineral mica, popped under high heat like popcorn. Holds water like a hard sponge. Common in potting mixes. Excellent for both clay and sandy soils because it both loosens soils and helps them hold water. Work into the soil before adding to the container or into the top several inches of the soil. It’s now harder to find in garden centres because of environmental concerns.

Perlite: Made of tiny, heat-popped volcanic rock. Holds water on the outside of the tiny rock. Often used like vermiculite; in addition, it’s an ideal medium for starting cuttings so they don’t get overly wet.

Fertiliser and Ironite

Plants need both major and minor nutrients. Minor nutrients (also called “trace elements”) are usually supplied by the soil, but plants use major nutrients in large enough quantities that we should add them regularly.

Of the major nutrients, nitrogen (N) is associated with green, vegetative growth; phosphorus (P) is associated with flowering; and potassium (K) aids root development. However, that’s an oversimplification. All three major nutrients are necessary, to some degree, for all aspects of healthy plant growth. If plants don’t have enough nitrogen or potassium, they’re unlikely to flower well, even though there’s plenty of phosphorus in the soil.

All different kinds of Fertiliser



There is no one best fertiliser formulation for annuals or perennials. Many work well. However, choose a fertiliser that’s higher in phosphorus and potassium than nitrogen to encourage blooming and discourage excessive leafy growth. If you’re growing perennials primarily for foliage, use a balanced fertiliser with equal parts N-P-K.

Fertilise annuals throughout the growing season, and perennials from spring through midsummer. Stop fertilising most perennials after midsummer so they’ll slow their growth and prepare for winter dormancy. Perennials that are still putting on new growth in fall will be vulnerable to winter injury.

Some newer annuals do better with frequent fertilising (every 10 to 14 days), particularly if they’re growing in containers. An easy way to provide enough nutrients is to mix slow-release fertiliser pellets into the soil when you plant. The pellets will release nutrients into the soil for about three to six months.

Ironite is a soil supplement

Ironite is a soil supplement and fertiliser used primarily on golf greens and athletic fields. Made from silver-mine tailings (what remains after ore is processed), Ironite contains iron plus secondary and trace minerals that help plants grow. A soil test can give you information about the nutrients in your soil. If your soil is deficient in minerals, you could work Ironite into the soil at planting time or add it later in the season. Despite some controversy, Ironite is widely available at home and garden centres.



The reason it’s somewhat controversial is that it contains small amounts of arsenic and lead (both as components of other minerals). According to Ironite’s producers, neither is released to your plants or to the environment, but some environmentalists and organisations discourage the use of this product.

What about alkaline soil and fertiliser?

Test Soil to Determine Nutrient Needs

Test Soil to Determine Nutrient Needs

Desert areas like yours often have alkaline soil (soil with a pH higher than 7.0). It’s easier to choose native plants that are well-adapted to local conditions than to fight nature to acidify your soil. Plant choices depend on where you live and how alkaline your soil is.

Alkaline-tolerant perennials include Russian sage (Perovskia atriplicifolia, Zones 4 to 9), globe thistle (Echinops giganteus, Zones 4 to 8), and Oriental poppy (Papaver orientale, Zones 3 to 9). For more choices, check with local nurseries.

To make soil less alkaline, incorporate elemental sulfur or sulfur-containing fertilisers such as ammonium sulfate or iron sulfate. Granular sulfur is cheapest, but also works slowest. In fact, reducing soil pH is always slow—it often takes several years. Your success will depend, too, on what type of soil you have. It’s easier to acidify sandy soils than heavy clay soils.

Incorporating organic matter such as peat moss or cottonseed meal will help acidify the soil. In a pinch, you can spray plants suffering from iron chlorosis (a condition in which high soil pH causes leaves to yellow and eventually turn brown, particularly between the veins) with liquid chelated iron. It will give them a quick green-up, but it’s a temporary fix.

Why duff is so important for the garden

In natural settings, leaves, bud scales, twigs, fruits, and flakes of bark all accumulate on the ground to form a layer of debris. Encouraging “duff” helps you feed, shelter, and protect wildlife. You can call this wonderful stuff “plant dander” if you like, but naturalists have called it “duff–an adaptation of the Scottish words “dowf,” meaning decay, and “deaf,” describing unproductive soil–for the last 150 years.

Duff is prized for its ability to:

Magnolia Overall

Magnolia Overall

Inhibit plant growth by separating seedlings from the soil or from the sunlight.
Reduce the loss of soil moisture through evaporation by preventing sunlight from directly heating the soil.
Protect the soil by reducing the impact of rain and hail.
Prevent erosion by absorbing and deflecting both runoff water and wind.



Duff is nature’s mulch, although it doesn’t look much like the man – made stuff. In the garden, mulch presents a uniform appearance. It looks tidy, but natural duff appears unkempt, although this impression is deceiving. In fact, for wildlife gardeners, having a bit of duff in the garden is something to aspire to.

Wildlife emphatically prefer duff. Synthetic mulch, despite the convenience it brings to traditional gardens, can be hazardous to wildlife. For instance, some birds pull strips from exposed woven – plastic landscape fabric and then use them as building material. This can lead to them becoming entangled in their own nests; nest failure and death inevitably follow. (Orioles are particularly vulnerable due to their unique style of nest weaving.) In addition, mammals sometimes eat plastic mulches, though we don’t understand why or how; plastic fragments in coyote, fox, and raccoon droppings leave no doubt of this, however. Also, salamanders, toads, frogs, and snakes can become disoriented and trapped beneath plastic sheeting.

Duff, by contrast, figures into wildlife habitat as both food and cover. Duff stores raw materials much like a warehouse does, patiently holding them until they are ready to be processed back into the community. Dead leaves, for instance, contain phosphorus, which is an essential element in the building of plant cell membranes. A dead leaf becomes the food source for recharging the soil with phosphorus, and the soil in turn feeds new plants which become the food source for animals.

Robins scratch aside its leaves to find earthworms; towhees do the same to find crickets. Thrushes search the duff to find snails, and sparrows inspect the cover to find seeds. Each species has adapted to find some portion of its food by scratching through the duff. Grosbeaks and buntings select nesting material from the duff, and ovenbirds nest in it. Frogs and toads prowl here, while shrews hunt and nest and sleep in the duff, as well.

Usually a valuable commodity, duff can turn into a liability in certain situations.

Duff Which Accumulates In An Oak

Duff Which Accumulates In An Oak

Thick duff can become a fire hazard. Duff which accumulates in an oak forest of the Continental East or a lodgepole pine forest of the Mountain West, can become a fire hazard; we learned this in the 1988 Yellowstone National Park fires. Occasional light fires minimise duff in such places.

Duff can also teach us how to distinguish forests from woodlands. Forests, which are communities where trees grow close enough together that their crowns form canopies, produce expansive and thick duff. Thin and patchy duff characterises woodlands, which are communities where trees grow farther apart in a more scattered placement so that their crowns cannot form canopies. Wildflowers, butterflies, and birds all know these distinctions, and therefore the wildlife gardener should know them, too.

Duff, as a community within a community, serves wildlife whereas mulch serves the gardener. To use synthetic mulch is to take in the wildlife welcome mat; to let duff accumulate naturally in areas is to put that welcome mat back out.

As you clean your yard and garden this fall, think of duff as something to encourage among your shrubs and perennials. If you leave plant debris in place instead of raking the soil bare, your wildlife will thank you for it.