Hi Chris, Sweet Williams are fully hardy plants although as yours are still quite small I would be inclined to give them a little bit of protection this winter. It would be best to plant them out now and place some horticultural fleece over them. This allows the plants to slowly put on root growth in warmer periods, which will give them a much better start next spring. Planting them in the ground will also help protect the roots from freezing. Horticultural fleece is available in all good garden centres (and some DIY stores) and allows light, air and water through so can be left on until next spring.
Alternatively you could leave them in their seed trays and place them in a cold frame or other sheltered spot in your garden. It's best not to leave them in an exposed position in a seed tray as the roots will be vulnerable to freezing, which could easily lead to death. As soon as they start to put on new growth next spring you can plant them out to their final positions and they should be fine without protection. With just a little bit of shelter from the worst of the winter weather your Sweet William plants should be fine! I hope this helps Chris.
Hi Nick, it is true that spent mushroom compost is alkaline in nature. Mushroom growers add chalk to the casing of their mushroom beds so any spent mushroom compost often has quite a high chalk content. Raspberries are prone to suffering from chlorosis in alkaline soils and are much happier growing in slightly acid soils. Chlorosis is where the leaves become yellow due to the plant being unable to absorb iron from the soil. It makes it difficult for the plant to perform properly and can result in stunted growth, poor fruiting and decreased resistance to stresses such as drought. Don't worry yet though as it takes quite heavy or frequent applications of mushroom compost to increase the soil pH significantly!
As you've only applied mushroom compost to the soil once, I would wait and see how your raspberry plants perform next year. It's very difficult to make a soil more acidic as the effects are often short-term. Hopefully the compost will have had little effect on your raspberries; in future make sure you use multi-purpose or garden compost, or well-rotted manure to improve your soil. If your raspberries do look chlorotic (yellow leaves with green veins) you can treat them with sequestered iron, also referred to as Sequestrene, to help green up the leaves and encourage strong growth. Applications are best made in the spring. Good luck Nick, let us know how you get on.
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