Twig: compost compacts, but don’t use too much

Organic matter gets used up, which is why soil levels in planter boxes slump over time, and we need to replenish it regularly.

Elle Grips and Foxgloves Works gloves, worth $83, this month’s Twig prize for best letter.

I have several raised vegetable beds filled with potting mix. I always use compost and mulch but it compacts down over time. What should I add?

PAULINE CHIARELLI, BY EMAIL

This is such an important topic. Organic matter gets used up, which is why soil levels in planter boxes slump over time. We need to replenish organic matter regularly, but equally there can be too much. Organic matter should be in only the top 20cm-30cm; below that, insufficient oxygen causes problems with decomposing. In raised beds use coarse, washed sand to fill up to the organic matter-rich layer, or use mineral additives such as zeolite, vermiculite, charcoal and rock dusts that have water and nutrient-holding properties without decomposing. For the full story on organic matter, see gardendrum.com.au/2014/09/16/omg-organic-matter-guidance-2.

After nine years of great mandarin crops from our faithful tree we now have split fruit dropping off at cherry tomato size. The tomatoes also split before ripening. Why?

AMBRA SANCIN, SYDNEY

They are victims of Sydney’s recent weather. When we have massive downpours and soil becomes suddenly sodden, fruit take up too much water and expand so quickly that the skins split. Unfortunately there’s little you can do about it.

While fishing on the Ord River I saw a creeper with royal blue, pea-shaped flowers I later identified as Clitoria ternatea or butterfly pea. The seeds germinated but I can’t get them to grow on. What are their requirements?

LORRAINE, MANDURAH, WA

Widely naturalised, this herbaceous creeper likes a tropical climate with summer rain. It grows on most soils, including clay. Sow seeds late spring. Growth is fastest above 27C and stops below 15C. Established plants will regrow after light frosts. It’s used as a cover crop to increase soil fertility, as cattle pasture and for revegetation projects.

Our ‘Little Gem’ magnolias are not thriving in their large pots, growing spindly and rangy rather than thick and full, although they have flowered. Would pruning help?

FRANCES MILLAR, BRISBANE

My observation is that these evergreen magnolias grow like that when they’re water-stressed in pots or dry soils. The species’ natural habitat is the edges of water bodies and swamps. Prune all over in early spring or prune individual stems behind spent flowers through summer. Repot, increase watering and apply a wetting agent twice yearly.

Send your questions to: [e-mail] or Helen Young, PO Box 3098, Willoughby North, NSW 2068. helenyoung.com.au. The best question for May wins two pairs of the best garden gloves you will ever own, worth $83. Fully washable Elle Grips and Foxgloves Works are from foxglovesdirect.com.au.

FLORA

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