How about considering allergy-free trees?

As I mentioned last week, in relation to Hebes, the time for planting bare-rooted evergreens is either Sept/Oct or March/April, whereas bare-root or pot grown deciduous woody plants can be planted any time during the dormant season, so long as the soil is in reasonable ‘nick’ which is basically saying so long as it is not sodden, frozen or covered in snow!

Before getting on to my main theme this week, I refer you back to last week when we had a very interesting resume of Mr Don’s new book in which he covers the planting of trees. Don’t put compost in the planting hole – he says!

Rubbish, say I. I’ve probably planted more trees, incorporating compost in the planting hole, than he has had hot dinners and I can claim a very high success rate!

He goes on to claim that adding compost will cause roots to grow round in circles. Balderdash!

That is getting mixed up with the ‘digging square holes instead of round ones’ Red Oak in spring garb Monoeious Birch

theory. It relates to planting pot-grown trees. When they are tipped out of the pot, the rootball may be wrapped round and round. In that case, it should be teased out by hand or using a sharp knife, drawing the blade from top to bottom, three or four times round the circumference – no more than a pruning exercise. The roots will then readily grow out.

Adding compost to the soil during the planting process along with a sprinkling of Mycorrhizal granules over the roots will enhance the chances of a successful outcome so long as the sides of the hole are broken down further during the filling in and firming process.

Now, since we are in the traditional tree planting season, how about considering allergy-free trees?

I must start by acknowledging journalist Colin Hambridge who has just published a report on this topic following a seminar organised by one of our major tree growing nurseries and featuring Californian Tom Ogren, a leading expert Juniper

on the subject of allergenic plants and their impact on people and communities.

He devised the OPALS® (Ogren Plant Allergy Scale) which is now used by, among others, the American Lung Association and the USDA Urban Foresters. The scale measures the allergy potential of all garden and landscape plants within a range of one to 10, the higher a plant’s rating the higher its risk of being an allergen.

For example, Autumn Acer rubrum Glory is rated at one, while Callistemon citrinus (bottlebrush) is rated at nine. More than 130 different criteria are used to develop allergy rankings for plants, each factor is either positive or negative and most plants will have a combination of positive and negative factors that are computed to determine their OPALS® ranking.

The main factor being assessed is pollen. The potency of individual pollen grains, length of flowering, the size, density, shape and weight of the pollen grains, together with average rankings in actual skin scratch, patch and sniff tests, and cross-reactivity to food allergies. Additional information regarding the factors used to build OPALS® may be found in Tom’s book Safe Sex in the Garden!

Tom’s main objective now is to help drive a robust, healthy trend toward allergy- and asthma-friendly gardens and landscapes in urban areas. For 30 years he has been researching the connections between the planted urban landscape and human health, linking pollen allergies and asthma. “Allergies affect how we perform in our lives and can have major negative impacts on us,” says Tom.

Fascinating stuff – how about this...Female trees do not produce pollen, just remember that when you go to buy plants especially if there is someone in the Dioecious Holly

family who sufferer.

Dioecious plants – those which are either wholly male or wholly female – are of particular interest to him. Dioecious trees include willow, red maple, holly, yew, juniper, mulberry and bay. Trees in the urban landscape, and especially those in public areas, are predominantly male because they do not shed fruit onto pavements.

On the other hand female trees produce no pollen is a possible and so may be termed ‘allergy-free’ or ‘allergenfriendly’.

It would be a much poorer landscape if we only planted pollen-free plants, but landscapers and designers should surely take this topic into consideration when designing their planting schemes.

■ If you want to know more on this fascinating topic, check out Tom Ogren’s websites www. allergyfree-gardening.com and www.safegardening.org