Blackspot, clivias and cordylines

I read that planting onions, chives or garlic beneath roses deters black spot. Is it worth a try, or will the onion smell over power the roses? Also does it actually work or can you recommend a method of controlling blackspot? I look forward to your comments.

Q: I read that planting onions, chives or garlic beneath roses deters black spot. Is it worth a try, or will the onion smell over power the roses? Also does it actually work or can you recommend a method of controlling blackspot? I look forward to your comments.

G. Price, Poringland

A: Blackspot is a fungal disease most prevalent in wet weather as it is spread by water-splash or wind-blown rain. Susceptible cultivars can be severely defoliated if left untreated. The disease is most prevalent in clean air areas and occurs less frequently where atmospheric pollution is a problem.

There are several non-chemical control methods, of which one is to plant the area under the roses with Alliums (onions) - however, it is essential to carpet the area with alliums, not just plant a small patch in one corner. So you may have more success by just removing infected and fallen leaves promptly and regularly. Hard prune infected bushes in spring and burn the prunings. Check out catalogues and plant roses resistant to blackspot, such as Rosa bracteata, 'Bonita' (ground cover) and 'Mermaid' (climber), but do not place heavy reliance on resistance because the fungus has several races and newly launched resistant varieties often soon succumb. Other varieties and Rosa species often show more resistance than climbing and bedding types.

Chemical controls should not be ruled out as they offer a quick solution to the problem. Spray with penconazole (Scott's Fungus Clear), flutriafol (Scott's Rose Clear Gun!: also contains bifenthrin for insect control) or myclobutanil (e.g. Bio Systhane or Roseclear 3 which also contains an insecticide) to control the disease, alternating these with the protectant mancozeb (Bio Dithane 945) to prevent the fungus from developing resistance to the fungicides. Avoid using products containing insecticides unless a pest problem is actually present. Several sprays may be required during the growing season, starting immediately after spring pruning.

Q: In the spring of 2006, when one of my Clivias was coming to the end of its flowering period, I left one stem on the plant and removed all but two of the seeds; I did this to see what would happen. These have now developed into red fruit about the same size of a cherry. Can you please tell me whether they contain one stone, as in a cherry, or several small seeds each? Also, do you know when they should be sown and how? Incidentally, this particular plant is now developing its flower spike whilst none of my other Clivias are showing any sign of flowering yet.

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Mrs S Parker Bungay.

A: Harvest seed from the berries now they have turned red and sow immediately. Sow singly in 7.5cm (3in) pots, using a loam-based compost (John Innes No 1) just covering the seed. Germination takes between six to eight weeks at a temperature of 21oC. Grow on at 16oC once the first leaves appear. The seeds are large and very similar to cherry stones. It may be worth nicking the outside of the seed to encourage the seed to take in moisture.

Once the seeds have started to grow Clivias are best grown in bright filtered light in a conservatory or on a windowsill. Avoid direct sun in summer which may scorch the leaves. Place pots away from radiators.

Pot up in a well-drained, loam-based John Innes No 2 compost mixed with multi-purpose compost (25% by volume) and grit. Do not plant too deeply.

For flowers to form, Clivias need a cool period of 10oC, from November to February. After this, water lightly applying a balanced liquid fertiliser weekly until the flower buds form, then move to a well lit position with a temperature of 16oC.

After flowering remove the flowers, unless seed is required, and reduce watering. Over winter water sparingly but do not allow pots to dry out.

Re-pot in early spring into a slightly larger container. Clivias flower best when well established in pots at least 20cm (8in) in diameter. Leave to grow on for several years undisturbed, top dressing annually with fresh potting mix.

Another method of propagating Clivias is to divide the plants after flowering into containers that just accommodate the roots. Water and keep at a temperature of about 16oC. Offsets can also be detached and potted up.

A common problem with Clivia is non-flowering this is usually due to over-potting or high temperatures over the winter. Plants that flower on short stalks usually they have had an insufficient cool period over the winter.

Q: I have two Cordylines in my front garden, which are now quite old (15 years) and have attained a height of some 22 feet plus. If I cut them down, will they sprout and grow again or must I get rid of them altogether?

Mr G A Young, Swaffham.

A: Cordyline trees are generally considered extremely delicate plants, but they flourish in the East of England. Established plants will tolerate a reasonable amount of frost, but the real killers are usually cold wet roots, or a cold damp head.

The plant grows on a single trunk that over time, often develops a distinct lean. Often, after flowering, the growing tip will divide producing a few short branches, each with their head of long thin leaves. Mature trees may develop into a classic lollypop shape. When these plants flower, the scent can pervade the entire garden giving rewards even when out of sight.

In my experience Cordylines are almost impossible to kill, you can cut them down to ground level any time now and they will start to regrow from the base in spring. Even where the plant has been completely dug up and only a few roots left they have regrown from those roots left in the soil. The problem you may have is they will send up lots of new shoots from the base so unless you require large bushy plants it is worth removing some of these new shoots to thin the plant.

Cordyline plants appreciate a fertile soil and a sunny position that is reasonably sheltered. Give them a good feed during the growing season, and before long they will reach flowering size.

The dead leaves often remain attached to the trunk and there is some debate as to whether to remove them or not. These dead leaves hold considerable amounts of moisture, which in their native habitat is possibly a life saver, but our damp climate should provide all the plants requirements from the ground roots. I personally prefer to remove them to leave a clean trunk. Do not tug the dead leaves. If they do not come away easily, cut them off close to the trunk.