Decision tree

for flood damaged farms
First few days

START HERE

Farm floods• Keep alive: look after family, stock and neighbours• Get cows milked and fed, clean up house and lookafter family and neighbours.
• Accept help

Is pasture green and growing?

NOSEE BACK PAGE

YES

Next week• Assess damage to access, fences and pastures (typeand depth of silt).
• Get soil test of silt2• Talk to bank manager and other resource providers.
• Accept help

Is pasture density OK?
NOYES

Is there a special need forPrioritise actions:1. Access to farm
2. Water systemwinter and spring feed that

3. Start on fences
4. Feed plan.

NOFeed plan5
Plan feed requirements /supplies given pasture damage.
Reduce stock numbers, consider drying off some stock,
get rid of culls. Devise strategy that meets short and long

can’t be met from regrassing
other paddocks?
YES

When soil is dry enough for

When soil is dry enough formachinery direct drill withmachinery undersow withperennial pasture species

short-term ryegrassof choiceof choice6

feed goals. Use decision tree below. Fertiliser strategy.
Call contractor, order seed.

Over next 6 weeks

Apply normal farm fertiliser

Implement regrassing plan. Continue repair ofinfrastructure. Document all damage and actions forpossible government assistance and for your own records.

programme with additional
immediate nitrogen to aid
recover of pasture.

START HERE
Is pasture green and growing?

NOYES

Is the pasture covered in silt or sand?1

Is debris present?1

YESNOYESNO
Mound up. Trees can be burnt.

SEE front PAGE

Does the soil smell?4
NOIs there a special need forwinter and spring feed that
can’t be met from regrassing

Is flood sediment less than 5cm deep?6
YESYESIs there a lot of thatch?
NONO

Is flood sediment 5-10cm deep?7
YESYESNO

other paddocks?
Mow thatch

Is flood sediment between
10-20cm deep?8
YES

Wait for soil to dry enoughNOto cultivate normally and sow
perennial grass/clover or shortterm ryegrass/clover mix

Is the deep (>20 cm) flood sedimentsilt, sand or gravel?9

Wait for sediment to dry andSandGravelSiltthen spread sediment evenly
on paddock. Use a form of deep
cultivation to bring buried soil tosurface. Is the resulting seed bed
Do you want to graze thismainly flood sediment?
NOarea in the next 2 years?

YESNO

Consider riverYES

bank planting
strategies toincrease organic
matter.11

Pile gravel in a big heap foruse on races and for sale.
Cultivate and sow remainder

Apply fertiliser to flood
sediment affected soils
according to expert advice
based on soil test2of paddock as normal for

Sow short-term ryegrass or

your accretion areas.

forage oats on heavy/medium
silt or accretion mix on sandHave the flood waters just receded

KEY:
Alive grassleaving silt moist with no sign ofdrying and cracking?

Oversow using coatedshort-term ryegrass seed.

10YESNO

Pasture dead no flood sediment
Wait for silt to dry. Then use light weight machinery and lightly break

Flood sediment damage

1,2,3

up surface, drill forage oats or short-term ryegrass and harrow behind.9... Additional notes over the page
Build up organic matter throughNote: In the medium-long term river silts are agrazing and mulching. Befertile, valuable resource and gravel overlays canprepared to recultivate in thebe managed to be cost-neutral.

following spring or autumn.

Regrassing flood sediment
1. Sediment structure: Flood sediment characteristically lacks

3. Pasture damage: Pastures that have been under water for 2-3

structure and organic matter and comes in varying textures

days will probably recover, but those under water for a week

ranging from heavy to light i.e. from clay to silty loams to sand.

or more in warm summer conditions will be dead. Paddocks

The clay loams are normally flat and feel smooth and silky andrecently grazed before the flooding will be worse affected

are usually found in ponding areas some distance from theas will pastures with at least 5 cm of silt cover. If a pastureriver. The clay loams retain water for some time and if deephasn’t shown signs of recovery after a week then consider

can’t be cultivated for considerable time because machinery

it to be dead. Better quality grasses and clovers will die firstwill bog. Silt to silty sands are often ruffled in surface texture

leaving plants with rhizomatous spreading habits – couch andand slightly gritty to touch. The surface of this medium floodbrowntop and creeping buttercup.

sediment dries fairly quickly but the subsurface remains moist.
Sand flood sediment is oftenrolling, gritty to touch and drains

4. Is the silt smelling? When river silts become smelly thisvery rapidly and is usually adjacent to the river and may contain

indicates that toxins are present as a result of anaerobic

a lot of gravel and flood debris.

organism activity. It is probable that seed germination afterAs a river floods, the coarser sand material is deposited insowing directly on smelly silt will be impeded by these toxins.

the higher reaches of a river and closer to the river bank andHowever recovery of the anaerobic silt following cultivation willthese will be of poorer quality – less fertile and less suitable forbe rapid. It is recommended that anaerobic silt be aerated or

pasture growth. Further down a river and further away from

cultivated.

the river more clay loams are deposited and in theory, containhigher nutrient levels. But the immediate fertility of the clay
and silt loams will be highly variable and should be tested.
Silt and sand can intermingle in layers making mixing a goodoption. The regrassing response will differ depending on thedepth of silt. Shallow silt (<5 cm) can be ignored, up to 10-15
cm silt can be cultivated and greater than 20 cm will either
need to be oversown or partially cultivated.
2. Silt fertility: The silt washed onto your land can either be

5. Plan recovery. Understand the feed requirements for theupcoming winter and following spring and summer and plan
pasture recovery practices accordingly. There is little point toputting the entire flooded farm into temporary pastures to findthat an identical feed pinch exists in the following spring when
the pastures have to be renewed again. Assess each paddock
for time under water, density of live pasture remaining after
10 days, silt type (sandy, clay/silt loam) and depth (<5 cm,5-10, 10-25, >25 cm). Using the damage information andfertile material from a neighbour’s farm upstream, or it could

feed requirements in the future, plan the recovery using

be subsoil washed from slips on steeper land. In any case it will

the appropriate mix of short term and permanent pastures.

contain very little or no organic matter, will probably have very

Then develop a time line for regrassing. Some paddocks will

low nitrogen (N) content, and may also be low in phosphate.

need immediate action while others will need to dry before

Sands will be the least fertile and the clay loams will be more

cultivation. Back up plans are needed if the contractor isfertile. Flood sediment pH level is different to that of existing

delayed or will only visit once e.g. is there some way to getpasture. Soil tests can be taken in wet silt as the sample ispasture established using own machinery or that available fromdried in the laboratory. However remember that when the soil

other farmers?

is wet less soil is collected per sample so more samples will berequired. It can take up between 3-14 days to get the resultsback.
Initial soil testing has shown silts from this flooding are pH high6’s, Olsen P’s 4-11, K very low, sulphur OK, ASC (anion storagecapacity) very low. Flood sediment will need potassium andphosphate fertilisers and nitrogen. Pure silt should not havelarge amounts of fertiliser in one dressing as flood sediment
has limited capacity to store fertiliser due to its low anion
storage capacity. Smaller more regular fertiliser applications will
be necessary. Seek expert opinion based on your soil test.
Analysis of flood sediment flowing under the Manawatu bridge
showed that the resulting soils that will form from this flood
will be highly fertile after the flood sediment has weathered.

6. Flood sediment less than 5 cm. If there is less than 5 cm ofsilt and the grass has been under the water for less than 3 days
then it is probable that the existing grass will come throughthe crust. Existing grass recovers much more quickly than sown
grass. If there are open parts in the pasture then these canbe undersown using perennial grass: clover mixes. If you are
expecting a feed pinch in winter and spring due to the flooding
and this can’t be met from other regrassed then as a special
case open pastures can be drilled with short-term ryegrasses.
Apply nitrogen fertiliser (N) at about 50 kg N/ha to assist
recovery, either as urea (110kg/ha), as ammonium sulphate (at200kg/ha), or as DAP (at 275 kg/ha). Avoid using more than 25kg N/ha down the spout if sowing seed with a drill.

7. Flood sediment 5-10 cm. If silt has completely covered

10. Oversowing is not an option for very sandy flood sediment

the pasture it will not survive and will need to be regrassed.

but is an option for clay/silt loams. Oversowing needs to occur

Normal cultivation methods will apply at these depths.

when the silt is still damp and sticky and must occur quickly

Cultivation is recommended because silt is relatively infertile,

once water has receded. Only use coated seed. Once the silt

will contain no organic matter or N, and have poor structure

has caked and cracked it is too late for oversowing. Relying on

making it prone to pugging. A barrier to water drainage may

rewetting the silt after rain to foster germination of oversown

also occur on the interface between the old pasture and flood

seed is not likely to work. Oversowing is a more risky sowing

silt. It is better in the long run to cultivate these depths of silt

method than cultivation or direct drilling, so use higher thanso silt and underlying topsoil are combined. Sandy silts will dry

normal seeding rates. If the silt is too wet, the seed may rot,

quickly and heavy and medium silts more slowly.

and if it is too dry then the surface of the silt will cake andThe resowing should have a high chance of success as it iscrack, and the ryegrass seedlings will struggle to grow primarynormal farm practice in moist areas of lower North Island

roots into the “new” soil. Once silt is dry seed may also blow

to resow pastures in early autumn March-early April. Whenaway and birds become a major problem because they cansowing the whole paddock it is best to decide whether to sow

land on the dry silt. Oversow with short-term ryegrasses.

short-term species, or perennials, as it is not always wise to mix

Stocking of newly sown areas should begin as soon as possible

them. Short-term Italian and hybrid ryegrasses will be preferredwithout pugging. Mulching before regrassing is a good option

where feed production this winter is crucial, or where weeds/

for building up organic matter.

fertility are expected to make it difficult to establish agood perennial pasture mix this year. Apply potassium and11. Revegetating shingle will be difficult. If grazing is required

phosphate fertilisers with regular dressings of nitrogen as there

within 2 years then the sand/shingle needs to removed andwill be little or no nitrogen cycling in the silt.

stacked in a big stockpile. It can then be used for races or sold.
Alternatively the area can be retired and revegetated using

8. Flood sediment 10 to 25 cm: In silt of this depth there willsuch things as blue lupins. However be careful not to build

deeper and shallower parts in the paddock. By levelling theup seed stores of undesirable species if you are intending to

paddock it may be possible to bring most of the paddock into

regrass at a later date.

the 10-20 cm depth category. Once the silt has dried heavymachinery can be used and deep ploughing (e.g. swamp

Information contributors: This information has been compiledplough) will help to mix the silt with the topsoil. If this ison behalf of MAF sustainable farming fund and Meat and Wool

successful then cultivate and sow as normal. If the resulting

Innovation using information provided from experienced farmers,

soil is predominantly flood sediment rather than topsoil thenresearchers from AgResearch and Massey University and rural

sow in either short-term ryegrasses or forage oats or otherprofessionals from Greenfield’s Communications, DairyNZ,

deep rooted short term crops and return to permanent pasture

Wrightsons, Ravensdown, Hills Laboratories, E-Lab, Agriseeds,

the following spring or autumn.

Agricom, Balance, QuinPhos, Pioneer, Hills Laboratory, Wilsons
and Keeling, Horizons Regional Council, Pyne Gould Guiness, and

9. Flood sediment >25cm. There are two options for deep clay/
silt loam flood sediment and these are either oversowing witha helicopter or cultivating the silt and drilling. In either casebe prepared to recultivate in the following spring or autumn
due to poor soil structure. If cultivating deep silt wait for thesilt to dry sufficiently to support machinery. Use light weightmachinery (including small tractors) and lightly break up
surface, drill forage oats (annual, good option from farmer
experience) or short-term ryegrass and harrow behind. After
winter grazing forage oats can be mulched to aid in organic
matter or made into silage in spring. On very sandy areas use
the same seed mix as you normally use on your accretion area.

Williams and Kettle.