3 minute read
Gardeners’ Question Time
Blackfly love the lush shoot tips of clematis, but are easily removed
Q What is this black stuff on the stems of my clematis?
Nicola Philpott, Norwich A CHRISTINE SAYS From your photograph it looks like the plant has an infection of blackfly. This can be removed by hand by just running your fingers up the stem. Some people will use a forceful water jet but on thin stems such as clematis I find it does not work. Unless you get a very bad attack the blackfly should not affect the flowers.
Old pots and logs give shelter to the slug-hungry common toad
Q How can I control slugs and snails without harming hedgehogs?
Nigel Rylance, by email A MATT SAYS Attract predators that will feed on them. Put stacks of pots and log piles in a damp spot for toads and ground beetles. Centipedes also like dark, moist places. And lay some corrugated sheeting in the sun for slow worms. You can also use nematodes to target slugs, put down beer traps and go on after-dark snail hunts.
Q Is it thrips that are singling out my onions at the allotment, shrivelling up the leaves?
Neal Candelent, by email A BOB SAYS Thrips are unlikely to be the problem: if it was, your allotment neighbours would suffer from these mobile pests as well. The same goes for onion fly. So, it’s probably a soil infection. If it was onion white rot you would have seen a whitish-grey fungus coating the bulbs. Rust would have caused orange spots and streaks, and downy mildew would have shown up as pale areas, becoming downy-grey.
Most likely it’s onion smut. The spores infect seedlings with dark spots and streaks before leaves thicken, twisting and distorting them. You should also see black spores forming on the bulb scales: these last in the soil for years.
Your best bet going forward would be to start your onions from sets in cells, and to plant them in different soil.
Soil-borne smut can be avoided by sowing onion sets into modules
Potatoes and tomatoes are in the genus Solanum, with similar fruits
Q Why did our potato plants produce fruits that look like tomatoes?
Q Should I only dig potatoes after the fower has died?
Mark Carr and Steven Jude, by email A BOB SAYS In the past, flowers on the potato plant appeared at about the same time as new tubers were forming. Thus, to lift a crop before the flowers died down would not have been sensible, as the new tubers would not have had enough time to swell up. However, more recently we’ve been breeding potatoes to give faster, earlier crops. Whereas most garden varieties used to be maincrop or late-maincrop, most are now second early or first early, and so are quick to make tubers.
Not only has this breeding pulled forward cropping, it’s also meant that potato plants spend less of their precious energy developing flowers. Some potatoes, especially the more modern varieties, flower very little. Victorian farmers reckoned on an extra ton per acre being gained by de-flowering their crops, so it’s clear this saved energy can make a big difference.
Caroline and Xander Jacob, by email
ACHRISTINE SAYS Most gardeners would say that potatoes are produced from tubers, and this is correct. However, to create a new cultivar of potatoes you need to raise the plant from seed.
Seeds of a potato plant are produced once the flowers have been pollinated and fertilised, resulting in the ovary (fruit) swelling to produce what to all intents looks like a tomato.
This fruit contains seeds, which if ripened, sown and grown on, would then go on to produce plants, which would have characteristics from both the male and female parents. Tomatoes and potatoes belong to the same family, but the fruit of the potato is poisonous.
