With spring upon us, landscapers around the country will be dealing with the effects of wet weather – but how many know how to properly mitigate them? In this article, horticulture expert Jon Muller goes through a few tried-and-tested methods
New Zealand experienced some very wet and wild weather in the first half of the year, especially up north. I have talked to several clients recently, who have had problems with excess water and drainage. There are several situations that will cause these problems:
If your client’s house and garden is at the bottom of a hill, there will be run-off from higher areas. If the water can’t move away fast enough, you will get a build up of surface water.
If your client is living on the flat near a river; it can back up and overflow onto their garden.
Get run-offs right
One situation that increases water run-off is hard surfaces. Every time a new housing development takes place, you will usually get an increased area of hard surfaces such as tar seal, paving or concrete. These surfaces pick up water and, hopefully, quickly direct it to the nearest stormwater drain. The extra water puts pressure on the stormwater system, or can run off onto neighbouring properties. A lawn area or garden is absorbent, thus reducing the likelihood of run-off.
What steps can you take to reduce flooding or ponding on a client’s property?
Install drains at points to intercept water before it creates a problem. For example, you can install a Novaflo drain at the bottom of a trench and backfill with drainage metal. Ideally, place the drain at a point where it will intercept water run-off. Then you can connect the Novaflo with existing drains, a soak pit, or into a waterway. You should always have a Novaflo drain in behind a retaining wall to intercept or divert water. If you are thinking of using a pump to direct stormwater to a higher place, check with the local council. It is possible that option might no longer be legal due to the possibility of electrical failure and back up of water.
Make sure all surfaces drain away from the house towards a stormwater drain. You can use slot drains and sumps with grills connected to a stormwater drain to divert water.
Create gardens and lawns to absorb excess rainwater and make sure there is a gentle slope on the lawns, so the water can drain away. You can also place drains and soak pits in lawns if they are prone to excess dampness.
If you’re faced with a wet area which is difficult to drain excess water from, you could turn it into a naturally formed bog garden, or a pond. A bog garden remains wet for most of the year and can form the perimeter of a pond.
Pond or water feature?
What’s the difference between a pond and a water feature? A pond has living plants or fish in it, while a water feature just has water in it.
I always like to discuss this difference with clients and the fact that both need maintenance. The number of times I have installed a pond or water feature and the client promises to maintain it, only to have it turn into a murky mess, makes me wary of suggesting them in the first place.
You also need to make sure the water features and ponds you build don’t leak. There are several products on the market that work well to seal the base if you are using concrete. Some pool sealants have had tried and true success, so check carefully! The other option is to use a liner.
Plants that love bogs
Bog or water plants can tolerate being inundated for more than 12 hours at a time.
Here are some plants you can use in boggy conditions, which can grow on the margins of a pond:
Shrubs and trees:
Cordyline australis, cabbage tree
Leptospermum spp, manuka
Phormium tenax, harakeke
Rhopalstylis sapida, nikau
There are emergent water plants that have roots in the soil under water, and their leaves emerge at the surface. Floating water plants float on the surface.
Nymphaea, water lily (emergent)
Iris ensata hybrids that are sterile (emergent)*
Lemna spp, duckweed (floating)
*Note plants like azolla or Iris ensata that aren’t sterile can get in NZ’s waterways, so be careful and check if any water plants are invasive before using them in ponds.
An alternative for boggy areas is to build a raised garden. This is especially useful if your client wants to grow vegetables, as you can backfill the garden with a well-drained fertile mix such as compost and good quality soil.
Jon Muller has owned Wellington Gardens Ltd for 24 years. He practices landscape design and construction, with an emphasis on planting design.
He has taught at polytechnics and university, mostly in soft landscape subjects. He has written six gardening books, including Wellington Gardens, and enjoys helping clients find landscape solutions. He also helps students and workers studying for their landscape qualifications.