Gardening tips this week include growing polemonium, dealing with carrot fly, and disguising those ugly-but necessary garden features.

Best of the bunch - Polemonium

These cottage garden favourites look great in dappled shade, where their lance-shaped leaves and clusters of cup-shaped blue, white or pink flowers show up well.

Also known as Jacob's ladder, polemoniums thrive in fertile, well-drained soil and don't take an age to become established.

Among the most popular is P. caeruleum, which grows to around 60cm (2ft) tall, producing soft blue flowers with bright yellow stamens, and self-seeds freely.

Try the white variety, 'Album' or the shorter, lilac type 'Lambrook Mauve'. P. foliosissimum is sought after for its lilac flowers and orange stamens, but is hard to obtain, while P. reptans is a spreading plant.

They make good companions with other early summer-flowering plants, including aquilegia, geum and trollius, and are generally trouble-free, although flowering will not be so prolific in poor soils.

Good enough to eat - keep carrot fly at bay

When the roots of you carrots are attacked by slim, creamy yellow maggots which tunnel into the flesh, leaving orangey-brown marks on the outside of roots, it's likely the larvae of the carrot fly is responsible. The classic sign of carrot infestation is crimson-red leaves, but leave them in the ground if you suspect damage.

One way of lessening the risk of carrot fly invasion is to sow your carrot seed after late spring, so you should miss the first generation of the larvae. Similarly, any carrots that are ready for harvesting before late summer should miss the second generation.

The adult fly locates carrots by a number of means, including scent. Avoid the need for thinning carrots (and thereby bruising the foliage which releases its scent) by using wide spacing when sowing - or sow a tiny pinch of seed at regular intervals.

There is some evidence to suggest that growing carrots between rows of onions helps minimise damage (probably by masking the smell). The deterrent is only effective when the onions are in active leaf growth. Once the bulb begins to form, the effect diminishes. Four rows of onions to one of carrots has been shown as necessary to offer protection

Alternatively, try growing your crops under a covering of well-anchored horticultural fleece. Then the female fly should not be able to gain access to the crop to lay her eggs. Carrot root flies hover close to the ground, so you could protect your crop with 30cm (12in) high mesh screen.

Also, try growing resistant varieties such as 'Flyaway', 'Maestro' and 'Syntan'.

Three ways to... hide unsightly horrors

1. Use natural materials such as willow or reed screening to hide plastic compost bins and water butts.

2. Replace manhole covers with pre-made grids, available from builders' merchants, filling them with pebbles, cobbles or carpet planting which can extend to the area around the grid to integrate the cover into the surroundings.

3. Position a focal point such as a big architectural plant elsewhere, to draw the eye away from an unattractive object.

What to do this week

:: Cut overgrown Clematis macropetala back hard once flowering is over.

:: Cut lawns at least once a week now grass is growing strongly.

:: Pinch out the tips of trailing plants in hanging baskets to make them bush out. Deadhead them as necessary.

:: Hang yellow sticky cards in the greenhouse to trap whitefly.

:: Plant out dahlias raised from cuttings.

:: Apply a mulch to the shoulders of parsnips to prevent the flesh from shrinking and cracking.

:: Remove suckers on fruit trees, rhododendrons and roses by pulling away, rather than cutting.

:: Plant out arum lilies (Zantedeschia aethiopica) which have been flowering in the greenhouse, or stand the pots outdoors for their summer rest.

:: If you didn't put slow-release feed granules in with your container compost, apply a liquid feed on a weekly basis to flowering annuals in pots.

:: Plant out young dahlias when all danger of frost has passed.

:: Thin heavy crops of plums so that fruit size is maintained and the branches are not put under excessive strain.

:: Remove and destroy strawberries with grey mould (botrytis) and protect ripening fruits from squirrels, birds and slugs.