Soil pH and Roses

Plants are adapted to different environments that foster different soil pH values - think of the oak-health woodlands of North America with Rhododendron, Kalmia and blueberries (Vaccinium) versus the parched calcareous cliffs of the mediteranean with Lavandula dentata. Plants also need nutrients and minerals in different ratios and the right pH allows for the uptake of adequate nutrients which become unavailable outside their ideal range.

The chart below is from the NSW Department of Primary Industries.

Influence of pH on Nutrient Availability

Roses like neutral to slightly acidic soil with a pH between 6 and 7

As you can see in the chart above most nutrients become less available as the soil gets more and more acidic - except for Iron. For instance potassium (K)- a nutrient very important for strong healthy cell walls becomes less available as pH drops below 6. This means that plants will be more susceptible to fungal diseases such as blackspot. Likewise if your soil is above 7 the availability of iron, copper, zinc and manganese starts to wane.

If roses are unhappy with your soil pH they will show with unhealthy leaves and lack of vigour. For instance a too alkaline soil might lead to interveinal chlorosis caused by iron deficiency. If you suspect this, testing the soil pH around the rose would be a good first step.

Also in some rose forums I have seen it mentioned that specific roses respond to different pH soil with slightly different colouration and also stronger or weaker scent. So if your rose was promised to be fragrant and is lacking scent, it might be worth testing you soil pH too. Most nurseries sell easy to use pH kits - I like to use this one developed by the CSIRO.

Ideally you will not have to try to modify your soil pH. I think the less interventions in your native soil and growing suitable plants is best. But if you want to grow vegetable crops or exotic ornamentals like roses and your soil pH is slightly out of the optimal range you can nudge it towards what you want with amendments.

In some parts of our garden we have suuuper acidic soil, with a pH of 4.5. I have amended the soil (at various times, not all at once) with garden lime, wood ashes (from our fire place) and home made compost, and heaps and heaps of mushroom compost and the pH is now closer to 6. We had 3 cubic meters of mushroom compost and 1 cubic meter of cow manure delivered by a local supplier Tunks . This is what that looks like:

I tested the mushroom compost and it has a pH of 8 (alkaline) I will have to keep monitoring the pH.

Hopefully you have the perfect soil, but if you are going to add lime to your soil because it is too acidic be aware that your soil texture will determine how much lime you need to add to shift the pH - as sandy soil is much easier to shift than clay you need much less. Lime and dolomite products differ so I am not going to recommend dosages - you need to follow the directions on the packet. Over applying lime can lead to other nutritional deficiencies as it puts other ratios out of balance and so it is better to apply only a little a time and wait and observe in between applications. Changing pH is a slow process!

If you have very alkaline soil you could try using leaf mould, spent coffee grounds and chicken manure and acidic mulches such as leaves and pine needles to help lower your pH. You can also use sulfur or iron sulfate.

NB: Before adding any amendments (or buying large quantities) I think it is important to first test their pH. Cow manure, for instance, is generally thought of to be an acidic to neutral soil ammendment, (and therefore often advised as a suitable amendment for roses and camellias) however I think it must differ a lot according to source and the cows’ diet. I tested three different local supplies of cow manure at a pH 8, and a bagged cow manure from my hardware store was also pH 8.

Some Soil Ammendments

ACIDIC

Pine needles

Leaf mould

Pine bark mulch

Coffee grounds, Spent tea leaves

Compost (without added lime)

Fowl Manure (more acidic) and possibly some Cow manure (without added lime)

Sphagnum peat

Elemental Sulfur

Iron Sulfate (Also called Ferric Sulfate)

Specialist Camellia fertiliser

ALKALINE

Wood ash

Stabled horse manure (lime added)

Mushroom compost

Garden Lime

Dolomite

Some cow manures

Selecting Roses for Blue Mountain's Gardens

If you like me, have been beguiled by all the frothy images of David Austin and old garden roses, you may be wondering if they will do well in the Blue Mountains. The answer is a resounding yes! However the key to armfuls of roses and healthy bushes is understanding your garden’s climate, soil and light patterns and selecting suitable roses and positioning them well.

So as a budding rose enthusiast I want to share some of the insights that I have gleaned through the writings of experienced rosarians and also from my experience choosing and growing roses in the upper Blue Mountains.

Things to consider when choosing roses:

Sunlight

Roses require full sun, which means at least 6 hours of sun per day. However I find some roses do not cope with hot afternoon sun, so a position that gets morning sun is ideal. Also as dew will evaporate quickly, morning sun will help prevent blackspot and other fungal infections. Doing a sun chart of map of your garden often reveals surprising results, with areas considered full shade possibly having more sun than previously thought and vice versa. The best way to do it is at various times of the year (say Spring and Mid Summer especially) put major areas of your garden on a chart and record whether they are in Sun, Part Sun or Shade at each hour of the day. This may seem like a time consuming exercise (you could do the same by setting up a time lapse camera) but will be invaluable for understanding your garden and possibly squeezing in more roses.

Although roses like full sun, some cultivars can do very well with a little bit less, and they will also perhaps hold on to their colour and scent a little longer. Roses in shade may grows bit taller, so you might need to prune a little bit more to keep them compact.

Roses that cope with partial shade (4-5 hours of sun per day):

The most shade tolerant: Rosa gallica ‘Officianalis’, Rugosa roses, Celeste, Cornelia, Ballerina, Iceberg

English Roses: Olivia Rose Austin, Roald Dahl, Princess Anne, Shropshire Lad, Lady of Shalott, Heritage, Abraham Darby

Climbers: New Dawn, Sombreuil, Zephirine Drouhin, Climbing Cécile Brünner

Sally Holmes, Cecile Brunner, The Fairy, Blueberry Hill, Bonica, Lavender Lassie, Complicata

High Rainfall

If you are in a high rainfall area and have rainy humid summers, like in Blackheath, choosing roses that don’t ‘ball’ (where petals in an opening bud stick together and the flower fails to open and rots) would be wise to avoid disappointment. I have found that deeply cupped roses with a very high petal count (those luscious cabbage-like roses) and thin petals are more likely to ball such a Souvenir de la Malmaison and some Kordes roses such as Earth Angel and Jasmina. I actually ended up removing my Climbing Souvenir de la Malmaison, despite the breathtaking beauty of her flowers, because the blooms were just too scarce and all but a few balled terribly. My Olivia Rose Austins do ball sometimes, however they produce roses so prolifically that I can just snip off the offending blooms and still have a good show.

Some of the rare roses of Souvenir de la Malmaison to actually open.

Some rain resistant beauties:

Single or semi double roses are a better choice than incredibly full roses. Examples: Nevada, or the stunning rugosa Roserie de l’Hay

English Roses that won’t ball: Princess Anne, England’s Rose, Desdemona, Tottering by Gently. Gertrude Jekyll is also supposed to be rain tolerant (it is my first year growing her, I will report back!)

David Austin Rose Desdemona

Soil

Roses are pretty tough and adaptable. But ideal conditions are a pH around 6.5, and soil on the heavier side, while still being well drained. This is because roses are hungry and thirsty plants and soils with some clay are better at holding moisture and nutrients (clay has increased cation exchange capacity) than very light sandy soils. The exception to this is Rugosa roses which actually prefer sandier soils, although they will tolerate clay as long as it is well drained. If you have either heavy clay or sandy soil amending your soil with organics such as compost, leaf mould and well rotted manure will improve drainage and also moisture retention.

Roses for Sandy Soil

Rugosas

Gallica roses such as : Charles de Mills, Rosa gallica ‘Officianalis’

Wild species roses.

David Austin also lists Claire Austin, Olivia Rose Austin and Lady of Shalott as ideal for poor soils.

Disease Resistance

High disease resistance (blackspot, rust, powdery mildew) should be at the top of your list of required traits if your garden gets misty or humid, especially if you don’t want to spray. A little bit of blackspot is nothing to worry about and usually can’t be avoided, however disposing of fallen leaves and mulching the ground thickly can help keep it at bay.

Kordes roses are exceptionally healthy!

Rugosas again are very disease resistant. Also remember never to spray the leaves of Rugosas with anything as they resent it.

David Austins with fantastic disease resistance: Olivia Rose Austin (completely blackspot free for me, even with no morning sun!) Munstead Wood, Claire Austin, Princess Anne. In my garden I have found that Wollorton Old Hall and Shropshire Lad get the most blackspot, although it doesn’t seem to effect the amount of flowers. I do feel that is it really important that climbers remain healthy as their leaves can’t hide behind other shrubs.

Brindabella Roses are bred for Queensland conditions and have a line of roses specifically bred for black spot resistance. I haven’t tried any yet but have my eye on a few! They would definitely be worth a try, especially in the lower blue mountains as I imagine they would cope with summer heat very well.

NB: If you do spray for fungus, make sure you use something organic such as Eco-organic eco-fungicide which is just potassium bicarbonate, as fungicide are some of the most toxic garden chemicals. You can make up a spray with some potassium bicarbonate, horticultural oil (eco-oil) (so that it sticks on the leaves), seaweed powder, and some liquid sudden impact or Charlie carp to give a foliar feed at the same time as you spray (using dosages recommended on packaging). Spraying foliage with just a solution of the Eco-organic seaweed powder is also a great preventative as it very high in potassium which is important for strengthening cell walls and resilience to foliar diseases.

Hardiness Zone

Hardiness in the Blue Mountains is generally not a problem for most roses as we just do not get cold enough to bother them. However you may be in a particularly frosty area or microclimate, and I would recommend avoiding doing your major pruning until at least early August .

Here in Blackheath, the average annual lowest temperature is -3.6°C, and this corresponds to a USDA hardiness zone of 9.

USDA ZONES

Using average lowest annual temperature.

You can find out similar data for your area using BOM’s climate data. Select monthly lowest temperature, then enter your location and choose the closest station and click get data. Scroll down and look under Summary Statistics for All Years. The lowest annual temperature is what you can use to find what USDA zone your area corresponds too. This USDA zone number is very useful when using sites like helpmefind.com, which will say things like “USDA zone 5b through 10b”.

Some roses (especially some old garden roses and species roses) need vernalisation or winter chill hours to flower well (just like some fruit trees like apples do) so that is also something to check if you have mild winters and are in a USDA zone of 10 or above.

Some areas in your garden may be hotter (heat reflecting off walls or paving for instance), frostier, or dryer (under eaves, near established trees, or next to walls). Observe you garden carefully. Keep this things in mind when positioning your rose.

Drought and Heat tolerance

Hybrid Teas in general need quite a bit of water to thrive and keep on blooming, in contrast once blooming old garden roses are likely to need less water to put out their annual show.

David Austin roses are notoriously thirsty! It makes sense as a lot of England has high rainfall. However there are cultivars that are better than others. Roses that are reputed to be drought tolerant will also cope better on sandy soils that dry out quickly.

Obviously you can do a lot to help your roses through drought - drip irrigation, deep mulching, compost amendments, soil wetter if you soil gets hydrophobic. Also avoiding very hot sites and areas exposed to hot midday and early afternoon sun will help when temperatures climb. If a very hot summer is predicted, it may be advisable to give your roses a prune instead of a deadhead after the first Spring flush. This will delay flowering, hopefully until after the hottest parts of Summer are over and save losing roses to crisping in a heatwave.

Rugosa roses and rugosa hybrids are drought tolerant and like sandy soils.

Roses with China heritage such as Mutabilis are also supposed to cope with not much extra water once established.

Tea roses (not hybrid teas) are said to be drought tolerant once established. Such as the classics Duchesse de Brabant (aka Comtesse de Labarthe)

Ramblers are generally pretty deep rooted and drought tolerant.

Mrs. B. R. Cant is reputably a very drought and heat tolerant and will also do well in Sydney.

Gangly giants and house Eaters

What might be a demure and well behaved shrub in Shropshire may become an octopus-tentacled monster with our hot summers.

Assume that many English Bred roses will exceed their size description. Some English Roses that stay compact:

Tamora (an apricot coloured rose with a Myrhh fragrance), Munstead Wood, Darcey Bussel, Boscobel, Ambridge Rose, Princess Anne

Ross Roses have actually tried to breed English Style roses that remain compact in Australian conditions- such as Lady Phelia, which is a compact version of Abraham Darby.

Beware Gales

Many areas in the Blue Mountains are really windy.

Don’t plant rose standards (roses grafted onto long stems) in high wind areas. Also smaller compact shrubs will withstand winds better than tall bean stalks (think of plants adapted to high wind areas like heathers) so if you have a windy spot pruning your roses so that they stay compact will reduce damage. Also some roses have more delicate foliage than others which is something to look out for if your garden experiences a lot of wind - delicate leaves will end up looking tattered quickly. Alba and Rugosa roses, have tough foliage and cope with exposed positions well.