Yarrow is a useful and versatile flowering herb. It has a number of applications for health and can be useful in a polytunnel or elsewhere in the garden as it will attract a range of pollinators and other beneficial insects which will help keep pest numbers down and benefit other plants grown nearby. The plants are also said to ward off ants and flies, to may help if these have invaded your polytunnel. Yarrow leaves can also be used, like comfrey, to make a liquid plant feed.
Yarrow will not like sitting in wet soil and does best in warmer, drier conditions, which means that the extra protection afforded by a polytunnel can be ideal. The good news is that yarrow will grow well on almost any soil from a light sand to a heavy clay (as long as drainage is addressed). Yarrow is particularly of note to those who have very alkaline soil, as yarrow is one plant that can do well in those conditions, though yarrow can also do well in acid, neutral and mildly alkaline soils. Interestingly, it can be planted on nutritionally poor soil and can be part of plans to improve the soil in a given area.
Yarrow is a dynamic accumulator and so will 'mine' nutrients from deep underground. It can then be used as a mulch or liquid feed to return those nutrients to the surface soil where they can be reached and utilised by other plants. Interestingly, yarrow can also clean up soils contaminated by lead, where it will gather lead before being disposed of.
Plants can be sown where they are to grow. Be sure to prepare the ground well before you sow your yarrow seeds to prevent competition. Plants should be spaced to a final spacing or around 1-2 feet apart for optimal growth, though yarrow will also do very well in a mixed bed or border, whether inside the polytunnel or near the doors. Since yarrow is known for attracting wildlife, it is of course a good idea to place it near other crops which can benefit from this.
While yarrow is in bloom, its white, yellow or pink flowers will attract pollinators through summer to early autumn. A wide variety of beneficial insects also find shelter in yarrow's fern-like foliage, using it as a habit for egg-laying or overwintering. Yarrows odour is also said to repel a range of pests.
Young yarrow leaves, though rather bitter, can be used as an acceptable addition to a mixed salad, though only in moderation. Yarrow is probably best avoided entirely during pregnancy, though has proven beneficial for women's complaints in the form of teas or tinctures. Yarrow is widely employed in herbal medicine for a range of different reasons. Harvest flowers and leaves for use as specified for herbal remedies, or simply enjoy yarrows pest-repelling powers.