Perennial wildflower habitats

Perennial or biennial wildflowers can only successfully be established on substrates where they can successfully compete with grasses. Fertile topsoils are not suitable. Subsoil, free-draining gravels, building rubble and other material provide sites which disadvantage grasses, and allow space for wildflowers to thrive and seed.

Subsoil

The topsoil is the dark, crumbly layer of soil, which can be anything from 25-50mm deep over sand or chalk, to a metre or more depth along river valleys and basins. The subsoil is the layer that lies beneath the topsoil and down to the underlying rock. Subsoil is very variable, but is divided here simply into those that are sandy and free draining, and those that are clay. Where the underlying rock or gravel deposits are close to the surface, the subsoil layer may be very thin.

Large sites, of a hectare or more, can be converted to perennial wildflower grasslands by stripping the topsoil. The sale of the topsoil may help fund the project.

Commercial operators will not be interested in small sites. Where the topsoil is a problem, first check the depth of the soil and the type of subsoil. Don’t attempt to remove an area of deep topsoil over waterlogged subsoil or you will merely create a pond! Choose the part of the site where the topsoil is thinnest, which will usually be on slopes or mounds. Re-use the topsoil within the site for tree nursery production, vegetable growing or raised beds. Local gardeners may be interested in buying small loads. note that the seedbank of mainly unwanted ‘weed’ species is contained in the topmost 100mm or so of topsoil. The lower layer of topsoil has far fewer weed seeds. Rather than removing the topsoil, a more feasible option may be to cover it with a layer of rubble, gravel or other material (see below).

On many urban sites, previous disturbance of the ground for building development will have left the subsoil exposed in places. Remaining topsoil may be covered with subsoil, or mixed in with it. Such areas may be suitable for establishing wildflowers as they are.

Seed mixes

These are normally supplied as a mix of 20% wildflowers to 80% grasses.

For clay subsoils, this is a basic list of suitable species, but there are many more which could be used. Choose those that are adapted to damp conditions.

The following is a basic list of species suitable for sandy subsoils.

Preparation and sowing

Sandy subsoils should need little preparation other than removal of larger stones or debris, and a raking over to produce a seed bed. Clay subsoils are more difficult, being sticky in wet conditions, and extremely hard when dry. Leave the subsoil exposed over the winter to break up the structure, and then preferably Rotavate and rake to create a tilth in the spring. A very thin layer, no more than 10mm deep, of seed-free topsoil (see above) can be used to make a seed bed as necessary. Sow at the recommended rate.

Management

Cut once in late September each year with a mower or strimmer, preferably removing the cut material.

Concrete and rubble

Concrete, brick and other building material can be used in various ways to create wildflower habitats. Always check first that any material on site or that you obtain is inert, and does not contain asbestos or any toxic material. Contact the Environment Agency (or equivalent) or local Health and Safety Executive Office for advice as necessary.

Rubble

Piles of rubble mixed with subsoil or other material may be suitable as they are, provided there is enough moisture- holding material to allow plants to establish. For small sites, the mound can be managed like a ‘rock-garden’, by sowing seeds or planting pot-grown plants into pockets of gritty soil within the mound. Cut with shears or strimmer after flowering, and again in autumn. Weed out unwanted seedlings, watching out in particular for ivy, bramble or other scrambling or woody species, which will rapidly shade out the wildflowers.

Chalk-covered mounds

Another technique is to cover the mound with crushed chalk or granular lime, and sow chalk downland flowers and grasses at the rate of 5g of seed per sq. m. These will thrive in the calcareous, free-draining conditions.Amound at least 1.5m high is recommended, to discourage the growth of deep-rooted rampant species. In the first year after sowing, the sward should be cut with a strimmer in late May, and again in late September, and the cuttings removed. In the following years, plants can be selectively trimmed with shears after they have set seed.

Crushed concrete

A further technique is to use crushed concrete, of a grade up to 10mm, spread over the subsoil or topsoil. Suitable crushed material should be obtainable from suppliers of hardcore. Crushing material on site will not normally be feasible, as old concrete is extremely hard. If someone wants a challenge, make sure they wear goggles and a safety helmet, and work well away from anyone else. Brick rubble is easier to crush, but will not produce the desired calcareous substrate.

First dig away the top 100mm of topsoil, in order to remove the unwanted seedbank. Then spread the crushed concrete to the original level, mixing it in into the soil beneath as you go. Rake to create a good seed bed, and sow a calcareous mix of wildflowers and grass, at the rate of 5g sq. m. Strim when the grass height reaches about 50mm, and remove the cuttings. Cut again in September. In following years, cut in May and September. Scarify with rakes to maintain an open sward to allow the germination of seedlings.

Motorway waste

During roadworks, the lower limestone layer of the road may be removed. The resulting ‘scrapings’, graded 10mm- dust, are very suitable for long-term wildflower meadows. The material has very low fertility, due to the high pH ‘locking up’ most nutrients, so that aggressive species are unable to survive. Sown with wildflowers only, the result is a very ‘open’ flowery scree, with plenty of bare patches where the flowers can seed. Establishment is slow, because of the low fertility, and it is important that any adjacent ‘weed’ species are not allowed to seed onto the scrapings while the wildflowers are establishing.

Supplies of suitable scrapings should be obtainable from motorway and road contractors. Avoid the top ‘planings’, which will contain a high proportion of tarmac.

Remove the top 100mm of topsoil from the site, and replace with motorway waste to the original level. To help retain moisture, cover with a layer of subsoil, no thicker than 10mm. Sow with a calcareous wildflower mix, at the rate of 5g sq. m, and rake it in. Because of the low fertility, a single cut each September will be sufficient.