Duration: 43 minutes
First broadcast:Friday 04 October 2013This week Eric Robson and his Gardeners' Question Time panel - Christine Walkden, Matthew Wilson and Bob Flowerdew - are at West Dean College and gardens, near Chichester.

Produced by Victoria Shepherd
A Somethin' Else production for BBC Radio 4.

This Week's Questions:

Q: I have a herb garden about 40ft (12.2m) square divided into four beds. Both beds were sown at the same time and regularly given the same attention and feeding, but there appears to be a strip 3ft (0.9m) wide running North-South in the Eastern bed where none of the herbs grow anything like as well. What is the cause?

A: It's quite likely that you could have a Victorian path underneath and this may have been sprayed with vicious weed killers in the past, which can persist for a long time. The nearby Eucalyptus routes will also stretch a long distance and could have a drying effect on it and it could be down to compaction. You should add organic matter; make sure it's well irrigated and dig deep to see if you can find the cause.

Q: Has the panel any ideas for getting rid of Mare's Tail?

A: It can be controlled but does need a great degree of perseverance over a number of years. You can cover it for a long time but this will need at least two to three growing seasons. There are also herbicides that can be applied but are not guaranteed to work. Generally Mare's Tail is not as bad on dry soils, but is far worse on wet soils. If it's in an ornamental scenario you could try growing plants that will thrive alongside it including Rodgersias, bigged-leafed Hostas and Bergenias. You should select plants that will run and spread alongside it and can outgrow it.

Q: Recommendations for planting close to the sea.

A: Look for plants that have got protective coating, a good thick cuticle and those that show a waxy texture. Those with silver foliage and hairy foliage tend to survive salt spray. It is also worth making sure the plant is established at planting, so apply plenty of organic matter so that it will grow a good root system. Plants that you could try include; Cranbury maritima, Glaucium flavum (the yellow-horned poppy) and Lizard Orchids.

Q: Does the panel have any special tips for successfully striking cuttings of the downy-stemmed plant Salvia leucantha?

A: Salvia leucantha has square, quite hollow stems so you should be careful when taking cuttings. You can strike them late and put them into very gritty, sharply drained compost and a layer of grit or Vermiculite at the neck because you can have problems with neck-rot. If you push them into florist foam or fiberglass cubes you should see good takes. You can then plant with the florist foam still attached and the plant will burst out of it when ready.

Q: Our garden seems to attract lots of cabbage-white butterflies which seem to favour our tomato plants. We get lots of caterpillars and they decimate the leaves. We tried a spray made from crushed-chilies and have also tried handpicking but we can't seem to get good results. Can the panel suggest other organic cures?

A: There were plenty of cabbage-white butterflies around this year but it's unlikely that their caterpillars are eating your tomatoes because they tend to stick to the Brassica family. Tomato leaves are quite poisonous and it is unlikely that they could manage these. It is possible that the caterpillars are the tomato-moth. Hand picking is best to get rid of them. Another method is to put newspaper down carefully and quietly under the plants and then give them a good shake which should make the caterpillars drop down and you can remove them.

Q: I have a very splendid Euphorbia mellifera growing in a sunny corner by my kitchen wall. It's five years old, nearly six-feet tall and smells wonderfully of honey in early summer. It is now getting a bit leggy and worn-looking. The RHS pruning manual advises keeping pruning to a minimum in cool climates. How can I keep my Euphorbia bushy and flourishing?

A: The RHS advice is good advice. The plant is not 100% hardy, so you should be careful because if you get your timing wrong and there is a hard frost it could kill it. However, if it has got leggy you might as well bite the bullet and prune it. You could take out every third stem and see if that gets enough light into the new foliage to encourage it to grow and if it does then next year you can do the same again.