Q: I have a pretty Nishiki willow that has been getting thinner each year in spite of clipping it back in summer. I've taken out several dead branches and given it 20-20-20 fertilizer in water. Are fertilizer spikes any good for this type of tree? Also, could snails on the tree be doing it harm?"
A: The Nishiki willow isn't always easy to grow well. It hates hot summers, dryness and clay - especially compacted clay. In summer droughts it needs lots of water but this should move freely through. That's why it's happiest in well-drained soil.
In poorly drained soil, some of the roots may rot and the fact you have some dead branches is a clue your tree may have some root rot already.
I wonder if you have a spot in your garden that has richer soil, less clay and is somewhat raised so drainage is better. Otherwise, just go on with what you're doing but try mulching your tree with Sea Soil or weed-free compost.
The 20-20-20 fertilizer will do nicely and several applications through summer should keep your tree nourished. The clipping should be helping but if new shoots are sparse or dying back in winter, you might try clipping and removing one-third of old wood in the dormant season.
All willows are easy from cuttings, though with the Nishiki willow cuttings will produce a bush, not a tree since most Nishiki willows from garden centres have been grafted onto a trunk. The Nishiki willows I've seen grown from cuttings are low bushy shrubs, but still glorious.
So if your willow shows signs of dwindling away regardless of what you do, you can propagate it from cuttings and enjoy it as a bush.
I should add that the Nishiki willow has a few other names including: Hakuro-Nishiki, Dappled Japanese Willow, Variegated Willow or Fuji Nishiki.
Q: Where can I buy astrantia plants and where do they like to be planted? How tall do they get?
A: The height of astrantia depends on the species. The tallest flower heads of Astrantia major grow about 60 centimetres-high while Astrantia minor is the smallest at 20 to 25 cm. The foliage of both species is a low mound of leaves. This can vary. Astrantia becomes taller or shorter depending on how rich and moist the soil is. It prefers semi-shade, but tolerates full sun if there's moisture in the soil.
Most garden centres have some varieties of Astrantia major. Flower colours vary from white and green, to white and pink to deep reds and rosy pinks.
Q: I have a one-year old Wasabi plant in a big pot. It is outside, loves this cool weather and is growing quickly. But where can I put it in hot weather?
A: You need to find the coolest, shadiest spot in your yard for your wasabi plant. It would be best if you put it on the north side of your house and blocked it from the late-afternoon sun. It will need to be watered a great deal, probably twice a day in hot spells.
The best wasabi is grown with its roots in cool, running water.
Anne Marrison is happy to answer garden questions. Send them to firstname.lastname@example.org