Plant Doctor Archive

Lack of pollinators and weeds in compost

I know there has been a lack of bees this summer, but I have also noticed a lack of female blossoms on my cucumbers, melons and zucchini. I was prepared to hand-pollinate, but with only one or two female flowers on the latter two, I was very disappointed.

My other problem is with weeds. Since bringing in compost and potting mix (I have tried several different brands), I have noticed a proliferation of weeds, some that I had not previously seen on my section. Nut grass, hydrocotyle, milkweed and numerous strange grasses and unidentified groundcover weeds. I've never seen paspallum here either - my lawn is all kikuyu - but there is paspallum in my cultivated areas. Are compost and potting mixes sterilized? Have other readers experienced these problems?

YOU seem to have had the same problem as me this year. The lack of female flowers on the plants is annoying, and there are a couple of factors that cause this. One is high rainfall. The high level of rainfall we have had can cause leaching from the soil of important nutrients like potassium, which is used for efficient flower development.

Rain also carries a large amount of nitrogen in it, so this can also cause an imbalance with nutrients. The way to counteract this is to feed the plants with a good fertiliser like Yates Thrive Flower and Fruit. It has a higher potassium rate, which encourages flowering and fruiting development. It has a lower nitrogen rate so you don't have a lot of leaf growth.

The introduction of weeds onto your property seems rather large, and could be caused by many things, such as wind or birds as well as unsterilised compost or potting mixes.

A lot of weeds will be coming into seed soon, so this will increase the number of new weeds germinating before autumn. Another common reason for the increase in weeds is due to you cultivating the soil. The seeds are brought to the surface and the light, warmth and moisture on them makes them germinate. Some seeds can last in the soil for many years, so this is why you get weeds or plants growing that you may not have had around your garden for many years.

When most composts and potting mixes are made, they are not sterilized with chemicals, but with heat that is generated in the stacks during production. These temperatures can reach up to 75°C.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.