Soil Is The Secret!

Very few groups start their community plantings with by replacing the soil in previously used containers, much less in entire in traffic islands. But think: last year’s plants have used up many of the nutrients in the old soil. The organic matter so essential for water retention has rotted down. Compaction from snow dumping and the winter’s deposit of sand, salt and cigarette butts have not helped. Without air, water and nutrients your plants have no chance at all against the heat reflected from the pavement.

No group is going to replace all the soil in every container, but it is possible to remove some of the exhausted growing medium and replace it with fresh.

Similarly few groups would renew the soil in a previously successful traffic island planting. Here a spring raking, cultivating, top dressing with compost and mulching are enough.

Below are some suggestions for soil mixes for new islands or containers, or for replacing soil in areas which have been unsuccessful in the past.


Be sure they have drainage holes. Over the holes put a layer of coarse material—stones, gravel, crushed plastic containers. Cover with fiberglass screening to keep soil from clogging drainage layer. Use a packaged soil mix for containers like Promix, Fafard, or PGM or create your own more economical mix from equal parts of coarse horticultural vermiculite or perlite, peat moss, compost, and topsoil.

Compost can be bought very inexpensively from recycling centers like Wellesley’s or Wayland’s.

Some groups like to add polymer products like “Soil Moist” to increase the water retention in containers. The dry crystals must be thoroughly mixed with the other dry ingredients and then slowly and completely saturated. A 20 gallon half barrel would require 4 ounces of “Soil Moist”; however, proportions of polymer to soil depend on the product chosen.


Excavate the soil to the depth of about a foot. Check the drainage by pouring a bucket of water into the hole; perforated drains may be necessary if the water will not go away.

Replace the soil with the mix described above, adding horticultural gypsum to counteract the effects of salt spray and bone meal for slow release nutrients. Ideally enough mix will be added to bring the finished level to an average of nine inches above the original grade in a gentle mound a foot high in the middle sloping to three inches at the edges; thus the total depth will be a minimum of one and three quarters feet or twenty-one inches.

Calculate the amount of soil you need carefully, as too much makes a mess on city pavement. Here is how: multiply the width by the length of the island. The depth will be 1.75 x the total. Example: an island 10 feet long by 4 feet wide is 40 square feet. It requires 40 times 1.75 equals 70 cubic feet of mix. Thus we need 70/4 or 17.5 cubic feet apiece of the four elements of the mix: vermiculite or perlite, peat moss, compost and topsoil.

Unfortunately, these items are rarely sold by the cubic foot, so you need to do more arithmetic. Useful facts about soil volumes:

Topsoil and compost are sold by the cubic yard (27 cubic feet); peat moss by the cubic foot; vermiculite and perlite as well as most packaged mixes by dry quarts or bushels (26 quarts to a cubic foot; 1 bushel to 1/1/4 cubic feet, and 22 bushels to a cubic yard.)


Mulch is both decorative and useful in maintaining moisture. If the top level is scraped off islands in the spring, a signficant amount of sand and salt will be removed with it it.

However, mulch has drawbacks which need to be taken into account. It should be no deeper than 3” and should not touch the stems of plants or it may rot them. Remember that it will steal nitrogen from the soil as it decays, so add extra fertilizer to compensate. Remember too that most mulches are highly acidic; add lime to the mix to compensate.

Mulch is usually sold in cubic yards. To calculate amount of mulch: 3” is 3/12ths or 1/4th of a foot. Since a cubic yard will cover 27 square feet one foot deep, it will cover 4 times 27 or 108 square feet to a depth of 3 inches.

Good luck!